I've read that in 1948 Jews and Jewish Organization owned only about 6% of the land. However to my understanding about 60ish percent of Israel is the Nagev desert. My question is at the time how much of what became Israel was actually owned land? Or to put it another way land people would live on and widely use.

I haven't seen specific tallies for this widely used, but I'll do my best to answer your question.

One of the first things that we have to note is what we define as "owned land". I assume you're not trying to include state-owned land, so I'll keep that separate. Another problem is what people would live on and widely use. That question, I don't think I can answer.

What I can answer is this: 60% of Israel is not the Negev desert. However, the UN Partition Plan of 1947 would have created a Jewish state that did have 60% of its land be the Negev desert, roughly. The UN Partition Plan, following the general Arab rejection after it passed the UN General Assembly, was not carried out. A civil war instead erupted, stymying any hope of carrying it out, followed by an invasion by the Arab states of the newly declared State of Israel on May 15, 1948.

The common 6 percent figure of Jewish ownership of the land privately arises from this report by the British, which comes from A Survey of Palestine. This number, of course, changed slightly by 1948: I've heard estimates that it went up to 9 percent. Even so, that is the vague idea.

The land itself, as you can see, was largely not owned by anyone. Jews owned about 6 percent in 1945, 6 percent was designated as "public land", and the rest was not assigned or owned by anyone and was also "state land". This book appendix asserts that roughly 70% of the land that became Israel was part of this "undesignated" or "public" land category, owned by the Mandate and not privately owned. That's the only source I know of on it, most estimates don't seem to talk too much about it. This map doesn't give hard numbers, but seems to reinforce the above ideas. The truth is, I haven't seen any hard numbers that say precisely how much of what specifically became Israel was privately owned, but those are the best guesses I can give with the information I've found.

Following up from above. How much of the Mandate of Palestine was Public land owned I suppose by the British Government and I'm assuming later turned over to the Israeli's?

I'm guessing this is based on the first answer and so it shouldn't need to be addressed separately.

How much of the land was owned privately by Arabs as opposed to by Jews.

Also, I believe answered.

As part of this I've also read that as per the Ottoman Land Code of 1858, many Arabs avoided actually registering land claims or outright misregistered them leading to a single person owning vast tracks of land on paper.

Indeed, this was a common problem with the Ottoman Land Code. It also meant that when the land was sold, for example, to Jewish groups who then hired new labor to handle it, a new class of landless peasants was created. Palestinians who had never "owned" the land in a legal sense but lived on it, thought they could continue to do so if another, wealthier landlord owned it. But when the landlord sold it, they suddenly had no option but to vacate as the new owner wanted different labor (usually Jewish labor).

if said person or family that was given a land claim to a whole village abandons his land was that considered forfeiture? or Forfeiting the title?

I'm not sure what you mean, but I think you're asking if land was abandoned in 1948, was that considered forfeiture? Is that what you're asking?