percentage of two numbers

hi this is probably simple but i can not work it out for the life of me.

what im trying to do is make a single float 0 to1 act as a percentage between two different numbers taking into account minus numbers as well.

so 0 will be original number 0.25 will be the original number + a quarter of the second number added or subtracted from the first if you get my drift.

can anyone help with this

suppose we want to fill a glass and want to know how much we filled

glassUnFilledVolume = (glassUnFilledVolume-dispanceAmount);