Sometimes, the pocket openings are dictated by the pocket irons.
For all of my customers, I cut corner openings at 141 degrees. and sides at 104 degrees. With that, I may vary the down draft angle, between 12-13 degrees, depending on a few factors (shelf depth of table, customer requests, etc..).
For pocket mouth opening, the typical request is 4 1/2" corners and 5" sides. Some owners want an even more generous opening, and opt for 4 3/4" corners and 5 1/4" sides.
The most common change that I make to change the difficulty of the pocket, is the hardness of the facings. My typical is 3/16" thick 60A neoprene. For pockets that really spit balls, I'll use 40A. I've never tried 80A, but I do sometimes use the canvas impregnated neoprene facings, if a generous playing pocket is desired.
As far as a template goes, good luck. It is mostly dependent on the table manufacturer, and the other work that has been done to the table. Ideally, the distance from the back of the featherstrip dado to the nose of the cushion would be 2". If that were the case, all pocket templates might look the same (or very similar). However, not all tables are built alike. Not only that, not all cushions are the same dimension.
The point is, every table is different. Cutting accurate and consistent pocket openings requires careful planning and measurement. Since you have an antique table, I would suggest that you start by measuring the placement of the diamonds. If the diamonds are consistently placed, you may use them for reference. To measure your pocket openings, determine the measurement from your diamonds, which will get you the opening that you desire. Don't forget the facings! With a 141 X 13 degree pocket opening, a 3/16" facing will measure about .25" (linearly along the length of the cushion).
Good luck!
