I tried masks, and I perceived that they help, but don't are absolute.
You need to evaluate the final result without them. It is the same with FWHM.
If you have refraction, the shape in masks and values in FWHM change continuously. They say: you are very near, and now it is your responsibility to accept the focus !
I prefer to use the FWHM, but as Canon 350D doesn't have live view ... With my Canon T3 I use FWHM.