I have studied quite a few dough recipes that call for the use of preferments. It can be quite confusing, especially with all of the bastardization of classic preferment rules that frequently takes place. But where there is a proven recipe, such as yours as noted above, where the preferment quantity is known and works, it is possible to express the preferment as a percentage of flour (a common approach among bread makers) or as a percentage of water (the Neapolitan method). If I did my math correctly, in your case the preferment is about 16.7% by weight of flour and about 26% by weight of water. Depending on how a spreadsheet or calculating tool is set up, any one of the three numbers--10% of total dough weight, 16.7% by weight of flour, or 26% by weight of water--should produce the same set of numbers.
When I first started using natural preferments, I used about 15-20% by weight of flour, as was recommended by member bakerboy. When I learned about Marco's methods for the Neapolitan style, I started using 1-5% by weight of water (but usually much closer to the 5% figure). In the latter case, the preferment acts as a leavening agent rather than as a preferment in the classic sense.