I would suggest you look at 20 bore modern shot-shell boxes, write down what those load "Equivelents" are. That will give you a good place to start. Start with, say trap or skeet loads and go from there into the hunting loads. Thus, 7/8+- oz and 2 1/4 +- drams is likely a good spot to start with. +- means just minor differences- exact is not important. Patterns are. With non-choked guns, some guys get better patterns with just using over-shot wads, not heavy wads- experimentation if your job to find what YOUR gun wants. Some guys get good results in non-choked guns with standard card and fiber wads.
Gr. Measure------Oz. of Shot
50-------------------3/4
60-------------------7/8
70-------------------1
80-------------------1 1/8
90-------------------1 1/4
100------------------1 3/8
110------------------1 1/2
120------------------1 5/8
For those interested in dram conversions- these are aproximate, usually rounded.
Note 1 pound = 7,000gr. 1 ounce = 1/16th of a pound - = 437.5gr. 1 Dram = 1/16th of an ounce = 27.3gr.
These are the charge weights as they pertain to old English as well as modern shotshell load designations on the boxes.
55gr. (54.6) = 2 drams
62gr. (61.5) = 2 1/4 drams
68gr. (68.3) = 2 1/2 drams
75gr. (75.0) = 2 3/4 drams
82gr. (81.9) = 3 drams
89gr. (88.
= 3 1/4 drams (edited to correct the actual gr. #)
96gr. (95.5) = 3 1/2 drams
102gr. (102.4) = 3 3/4 drams
109gr. (109.2) = 4 drams
116gr. (116.0) = 4 1/4 drams
123gr. (122.9) = 4 1/2 drams
130gr. (129.7) = 4 3/4 drams
136gr. (135.5) = 5 drams
143gr. (143.3) = 5 1/4 drams
150gr. (150.2) = 5 1/2 drams
157gr. (157.0) = 5 3/4 drams
164gr. (163.
= 6 drams
191gr. (191.1) = 7 drams
218gr. (218.4) = 8 drams
246gr. (245.7) = 9 drams
273gr. (273.0) = 10 drams
300gr. (300.3) = 11 drams
328gr. (327.6) = 12 drams
I have shot up to 12.1 drams in my .69 as an "accidental proof load" with one tightly patched round ball.
That was not fun, nor was it repeated.