From Thumb till middle finger 4 13/32 and 4 7/16 till ring finger - this span was done without the wrist-guard that helps my hand cup.
How much should i reduce to use a wrist-guard? I feel my hand stractched and causes blisters in middle finger.
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Try the wrist brace on, and put hand in ball.  See how ball fits.

I like to have my last crease 1/8 of an inch from back of ball.

This translates to a line drawn at the midpoint between the first and last joint which should just lay on the front edge of your insert(if you use one) or the edge of your fingerhole if no insert then you are correct.  Me guesses your middle finger span is too long with your wrist brace on.

REgards,

Luckylefty
PS above is with thumb in ball and hand comfortably stretched.  Above comes from Bill Taylor guru of ball fit!  Not me!

Jose -- Go back to your driller and ask him to refit you with your wrist brace on.  It's hard to estimate what the change will be, because each type of brace is different, and even within the same brace the change can be different depending on how much wrist cup you set the brace for.  That's one reason I like to watch my customers bowl before I fit them.  --  JohnP