As far as the length of the span, put your thumb into the ball completely and lay your fingers overtop on the finger holes, not stretched, but comfortable. The first joint of your fingers should be somewhere near the center of the hole. Most people have a span which is too long; when this is corrected it takes some time to get used to the new span (and generally does feel short).
Strictly speaking, more left lateral pitch (assuming you are right handed) will cause you to raise your track or decrease the tilt of the ball. For a starting point, find a plastic bottle (I find that the 16.9 oz aquafina water bottles are about perfect), and comfortably grab around the bottle. Do this a couple times and check where the center of your thumb nail is pointing in relation to your fingers (Mine points between the index and middle finger). The general rule is as follows:
Index Finger --> 1/8 L
Between index and middle --> 0
middle -- 1/8 R
between middle and ring -- 2/8 R
Etc
Etc
This gives a good starting point for lateral pitch.
For forward and reverse pitch, it is based upon span. A span of 4 1/4 should have generally around 0 pitch. Longer than 4 1/4" goes reverse (every 1/4" increase in span correlates to 1/8" in pitch) and shorter than 4 1/4" uses forward. There are some other factors such as thumb length, etc, etc, but generally speaking...
The formation of a blister on the back of your thumb can be a result of many things. As mentioned above, possibly knuckling or slightly bending the thumb in order to keep grip of the ball. This can result from the thumb hole being too big OR too much reverse pitch in the thumb hole...or muscle in the swing. I would start simple and go from there. Make sure hole fits properly..snug enough that very little pressure keeps the ball on your thumb, but you are able to still exit cleanly. If this doesn't work, look at the pitches. Maybe you need some additional forward to help hang onto the ball?
I would try those ideas first...
Good luck!
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