Speaking for Medieval Europe-- Often, they just didn't! (or at least didn't include measurements in recipes). For example in the Forme of Cury, recipes from Richard II's cooks, there are extremely few precise measurements. Two more from the next century are similar: it will provide phrases like "not too much," "a good quantity," or one of the most specific ones I've seen, "...caste a ladel-ful, or more or lasse, of boter þer-to, an a good quantite of whyte sugre, and a litel salt..." with a ladle being about as precise as measurements get, but also saying a little more or less is fine.

Now, it's likely that competent cooks of the time would be able to read the recipes and estimate ratios and amounts based on their experience, but certainly dishes didn't seem to be as precisely explained as today. It allows a lot of room for customizing to taste, though!