Munsonball's remarks about the pitch relating to span are correct as far as they go. However, you also have to consider the flexibility of the thumb along with the span. A 4" span, for example, can range from 3/16" forward to 1/16" reverse and still be considered correct. A good way to roughly evaluate the matter is to consider the grip(span+pitches+holes) as a pie chart formula. The span makes up 40%, the pitches 40% and the hole size 20%. The thumb pitch is 40% and this relates to 61.66 degree angle.If the relationships are not 40-40-20, the grip might still be acceptable, but have the potential for causing trouble, such as excessive callus formation and joint pain After considering flexibility you have to look at the length and moistness (or dryness) and size Additionally, although Bill Taylor
doesn't believe in side pitch, if you do drill in some side pitch the span has to be shortened or lengthened. As for your comments about drillers not knowing what they are doing, you may have a point...however, if you seek out an IBPSIA CertifedTechnician, you will find that he or she will probably be quite competent.I would also submit that most bowlers do not know as much about ball drilling as do experienced ball drillers, and your best bet is to see an experienced driller, preferably one who is at least an IBPSIA member.