Of coarse he can feel 1/8" difference. Most people can feel 1/16" difference, with a few even 1/32".
Ratpack, there are 4 things you can do:
1. Do as jkiser01 suggests....or
2. I don't like the big 1 3/8" slugs with an off center hole in my ball. Doesn't really hurt anything or effect the balls performance, but the looks bothers me. Mentaly I guess (lol).....Anyhow, if it was me, I would make the driller (if it was his fault) plug the thumb hole and re slug it at no cost to you with a 1 1/4" slug. He should be able to match the color close enough that a person will never know it's there when it is only 1/8" on one hole only..or
3. Ask the driller to either reduce the price of the ball plus plug and reslug at his expense or replace ball with a new one...or ask for a full refund. If he refuses any of this, report him to your local Better Business Buerau, Chamber of Commerce, small claims court, and etc.
4. Last, depending on your thumb pitch, if you have reverse pitch in your thumb hole, reslug only (at drillers expense of coarse) and try reducing it 1/16".....or if you have forward thumb pitch, try increasing it 1/16". If you have no trouble getting out of the ball, this might work.
If it comes down to reducing price of ball, I would not settle for less than 20%
For example, my driller charges $225 for an X Factor series Storm ball. This ball cost him $145 max. and possibly as low as $135. So, he has a mark up of $80 on the $145 ball and $90 on the $135 ball. Even after he refunded you with 20%, He would still make $35 on the $145 ball and $45 on the $135 ball. Not enough to survive running a proshop if he sold every ball this way. However, in this case we're talking about only one ball that was the drillers fault (if it was his fault and not yours) and this one ball won't put him out of business.
Now if it was your fault, you can choose one of the methods above at your expense.
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Don't wear your self out cutting wood with an axe...Use a Buzzsaw