If the turkeys have been fed on a reasonable feed schedule and feed, Then usually the best feed to weight to money spent for double breasted is around 20 weeks.
Weight can and does very, but if you feed about 80 % bagged/ custom mix and the rest free range, or 100% bagged/ custom, you more or less will be in this time frame.
You can butcher at around 15 weeks a still get some meat but they are still developing bone structure, and if you want a big turkey let the males go longer about 30 weeks is about a 40 to 50 pound dressed tom for great whites and BB Bronze.
If you use live weight, the figure about 30 to 33% of the live weight will be the finished weight.
Using the breast method varies from breed to breed on what you are looking for.
In order to learn how to check the breast the first year use the recommend time frames, then check to see how the breast feels. Great Whites and BB bronze have larger breast and can be a little confusing the first times you check.
If you have never checked the breast for size, or butchered a turkey before. you might mistake an air filled crop for breast meat. So check down towards there legs.
Tom