I detailed above how to determine how much to remove to achieve specific timing numbers for the various ports. That is one method, and others will use their own methods and achieve the results they are striving for.
As for specific timing numbers; they vary from saw to saw, and by what the person doing the mods is attempting to achieve. A very general starting point for piston-port saws is 100/120/78 (exhaust/transfers/intake). That is not the Holy Grail and is only somewhere start for someone with no idea. After that, achieving the results you desire is simply trial and error. The experienced porters have a good idea what numbers work for popular saws when trying to achieve certain manners in a saw.
And on top of that are port shapes and directions and sizes. And then there are strato saws which introduce another set of ports to consider.