I have a relatively simple case, however tricky.

I am testing a new treatment vs a control.

This is a new treatment that you do on one's eye, and suppose to decrease an outcome Y. For each subject, I use the treatment on one eye and the control on the second.

Pre using the treatments, Y is not exactly the same in both eyes, so when I compare how good the treatments are, I can't use an absolute value.

I wanted to set a paired t-test, of the % of decrease.

For example, if in one eye Y was 40, and now it is 29, there was a decrease of 27.5%, while on the other eye it was 41 and now it is 13, a decrease of 68.29%. The difference is then D=68.29%-27.5%=40.79% for this subject. Let's say I have N of them. I want to check the hypothesis that the mean of D is 0 vs. not 0.

Is what I am doing makes sense ? Any other ideas ?

Thanks !