Jakob
If the numbers you show refer to the weight of leather in ounces, the following should help.
I think my math is correct. 15/17 is 15/64 to 17/64 of an inch or 5.95 to 6.74mm
20/22 is 20/64 to 22/64 of an inch or 7.93 to 8.73mm
26/28 is 26/64 to 28/64 of an inch or 10.31 to 11.11mm
Tom