It is an individual issue..  Mostly decided by anatomy.

If a person has a lot of flesh in the intersection between their thumb and index finger they typically need more bevel than if they have no flesh there.

In that area is nerve bundles that on some people are out in the fleshy area or in.

Also if a person leaves span the same and increases forward pitch or decreases reverse with same span they will often have to increase front bevel*under flat part of thumb) to accomodate this sharper angle at the point of thumbhole ball intersection.  

Same guy yet more bevel than when he had more reverse or less forward on the thumb!
Otherwise one can end up damaging nerves in teh thumb.  If you have black marks or an electric feeling pain down thumb after increasing forward just add a slight amount of front bevel and pain goes away.  If an electric feeling stop immediately as you may cause nerve damage.

Note...side bevel does NOT need to be changed if lateral is left the same.

REgards,

Luckylefty
PS lastly....most proshops use a little machine called a bevel sander ...this usually increases bevel under both the front and side...don't do that if only going forward!  Use a bevel knife and sandpaper on front only and blend into side bevel.  finally use the mo Pinel tests to determine how much on each surface...front and side.
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Proud owner of a 140 first game average this year!  Don't bowl me the second two games though!  (two warm up balls in league..thanks proprietors)