The decision to go "Europe First" was definitively made at the first big meeting between the US and Britain, ARCADIA, which was held in December 1941. Roosevelt had been wanting to get into the European war for some time, and he and most of the Chiefs of Staff saw Germany as more dangerous. Every major meeting after that confirmed the principle of Germany first. But the US consistently insisted both in ARCADIA and all other joint meetings afterwords that pressure be kept on Japan and that offensives not stop entirely. The most frequent numbers bandied about in these meetings were 30% or 1/3.

But then as now it is hard to define how much actually did get divided. And a lot of that is because of how different the theaters were. Virtually no tanks were sent to the Pacific... not compared to Europe. And Europe and the Atlantic only got a fraction of of the Naval allotment of the Pacific. But it seems reasonable to say that they at least stayed true to the concept of the idea. At Germany's surrender the US had around 66-68 Army divisions in Europe, and 28 combined Army and Marine divisions in the Pacific.

I don't have a good book dedicated to the question of division of effort itself. But Ronald Spector's Eagle Against the Sun covers it and the conferences pretty well.

Edit: Corrected division counts.

why did they choose a Europe-First strategy?

The decision to concentrate the primary US war-effort on Germany (and Italy) crystallized in mid-1940 after the fall of France, when two factors were acknowledged by planners:

• The continued existence of, and 'friendship'/cooperation with, the British empire was the single most important factor in the strategic defence of the US.

• Britain was not strong enough to defeat Germany on her own.

Admiral Stark, in the 'Plan Dog' memo (October 1940):

if Britain wins decisively against Germany we could win everywhere; but if she loses the problems confronting us would be very great; and while we might not lose everywhere, we might, possibly, not win anywhere.

At the outbreak of war in Europe, US strategic planning was focused on limited engagement in the European theater and taking the offensive in the Pacific and Asia - this somewhat assumed that British and French resistance (supported materially by the US) was sufficient to protect US interests in the western hemisphere from Axis aggression (plan: RAINBOW 2).

Contingency planning for the defeat of both Britain and France dictated that American forces would be almost totally devoted to the Atlantic theater, and could potentially be facing a Wehrmacht bolstered by captured British and French fleets, which was estimated could potentially conduct military operations in the Americas within six months (RAINBOW 4) - so in this sense, any potential offensive against Japan in the Pacific had as a prerequisite condition the decisive defeat of the Axis threat in Europe (not so much final defeat of both Germany and Italy, but the potential they could dominate Europe and Africa and break-out into the wider Atlantic).

Louis Morton:

The critical point at issue in the discussions was the fate of the French Fleet and the future of Great Britain. The military wished to base their plans on the worst of all possible contingencies-that England, if not the British Empire, would be forced out of the war and that the French Fleet would fall to the Axis. The President, on the other hand, believed that American action should be based on the assumption that Great Britain would remain an active belligerent and that the military situation in Europe would not alter appreciably in the next six months. He did not feel, either, that aid to Britain should be cut off entirely, and countered the planners' arguments with the observation that if a small amount of aid would see the British through without seriously retarding American preparations, then that aid should be furnished. Nor was the President willing to put the armed forces on a wartime basis or to support full mobilization of manpower and industry. He agreed on the necessity for defense of the Western Hemisphere and the protective occupation of European colonial possessions as well as other strategic positions in the Caribbean area and in Central and South America, but only after consultation and negotiation with the other nations concerned.

There was also a line of thinking which supposed that Japanese aggression hinged on the success of German campaigns, and the relative ability of the European powers to protect their interests in the Asia/Pacific:

The main problem was how to avoid a conflict with Japan and at the same time maintain American interests and defend American possessions in the Far East. The answer perhaps lay in Europe, for there was strong reason to believe that Japan would take no overt military action against the United States or Great Britain until German victory seemed assured. This line of reasoning served to strengthen the view that as long as Great Britain was in danger, the United States should remain on the defensive in the Pacific. It was also a powerful argument for continued aid to Britain and for opposition to any move that might risk serious hostilities with the Japanese.

Effectively, the 'Germany first' decision had already been made before the Japanese attack. Ultimately, the American war played-out much like RAINBOW 5, with Britain dominating the Atlantic and Med navally, Russia taking the place of France as continental spoiler/cannon fodder, and a full-blooded thrust by the US on two fronts once the threat was neutered delivering the coup de gras.

You can read about this all online at the US Army Center of Military History (.com), a good start is Command Decisions. The division of effort between theaters is a question I wont really tackle (time!) - I recommend the two volumes of Strategic Planning for Coalition Warfare found here and here. A good indicator of the proportion of effort being applied in any particular theater at any particular time is the allocation of transport shipping and landing craft, which were strictly limited in quantity and necessary for any strategic offensive.