Back to the OP’s question. A while
back I was trying to better understand relationship of pocket size & facing angles and burnt up some time reading various old threads on the subject, where RKC and others had provided lots of info. The following is what I gleaned.
WPA & BCA specs for horizontal pocket facing angles are 142 deg +/- 1 deg. Diamond procut tables come at 4.5” mouths & 141 deg which most folks feel play perfect. But 141-142 deg is for pockets down to about 4.5” mouth size. RKC has said that pockets change by 1/8" per each angle degree change; eg a 4-5/8” pocket with 143 deg plays about same as a 4-1/2”, relative to the standard 141-142. The take away is that tables w/pockets in the range of 4.25-4.50” are best with 141 deg. 4.0-4.125” pockets are best with 140 deg angles.
Following that same logic, if one wants 3-3/4” corners that will still accept balls decently, you should subtract 2 more degrees, so you should be looking at 138 degrees. Which jives perfectly with the advice you got from
@rexus31 earlier in the thread.
Personally, I find pockets smaller than 4-1/8” change the game too much and aren’t actually that helpful for practice. Not so much because of the pocketing issue (obviously requires cleaner aim/stroke which is fine) but more because there is virtually no cheating of the pocket and one has to play quite different angles with the CB to get around the table.
If you are going this deep into very tight custom pockets, there are two things which I think get overlooked in these size/angle discussions.
First issue is that while modern pro tables seem to be somewhere in the 4.0-4.25” range, they always have brand new cloth; new cloth allows balls to slide in easier off the rails, so even though these pro tables are tight they don’t actually play as tight during the event as it may seem. Most of us with home tables and even most poolhalls are going to keep cloth around for at least a few years and the tables will play tougher as the cloth wears.
Second issue is that most pocket angle discussions focus on horizontal angles. But down angles play a big part also. WPA specs are a range of 12-15 deg, Larger angle funnels balls in more, but is closely related to shelf depth. Diamonds have deeper shelves & use 15 deg. BWs have shallower shelves and use 12 deg - in theory achieving roughly the same net result. So 12 deg down angle on a table with a deeper shelf might deflect more balls out than you’d expect. These subtle differences can have impact on how a tight pocket will accept balls hit with any speed, rattle factor, etc…
I’m no expert- the above is just what I’ve managed to learn from my research for my own table and playing around on various tables with tight holes. Hope it helps