Alfredo is right! The subtraction is back to front - go back to your primary school notes on long addition and subtraction!

I still think the method you're using to do the dimmer is flawed.

something for you to try:

when you get a zero crossing, this causes an interrupt
and loads the timer with the required value (as got from your lookup table). when this timer times out, the overflow interrupt turns on the triac (or whatever you're using to control the output). This way you're not sitting in a dead loop controlling the light. You have to make sure the zero crossing detector gives you a pulse on both the positive and the negative zero crossing. Another thing to note is that the triac optos and the triacs need a little time to turn off - allow 500us before and after the zero crossing for this.

When I find things aren't working the way I expect I use a simulator and single step the code - you'll see what your code actually does vs what you want it to do.