A few examples:

Many folks feed their starters equal weights of water and flour. So say you were feeding your starter and used 50g of it after wasting some:

50g starter = 25g W + 25g F

now you add 25g W + 25g F

Total = 50g W + 50g F = 100g

For the preferment calculator:

(weight of water/total weight of preferment)*100 = (50/100)*100 = 50

Say you keep a stiffer starter:

45g starter = 20g W + 25g F

now you add 20g W + 25g F

Total = 40g W + 50g F =90g

For the preferment calculator:

(weight of water/total weight of preferment)*100 = (40/90)*100 = 44.4

It doesn't need to be exact if you are using relatively small amounts of starter! Like 7g of it.