Their really is no set formula for Gambrel roof angles some might think so, but all kinds of combination’s are used and it not very hard to find many different examples.. and some with not so good results..
One type rule is that the lower steeper roof pitch is always a 12 inch rise in every 7 inches of run or, a 12/7 roof pitch and the top roof pitch is always a 7 inch rise in each 12 inch run.. or a 7/12 roof pitch, and also make both rafter lengths about equal.
Another rule of that is used is always use 45 degrees or a 12/12 roof Pitch on the lower section and always a 22.5 degree or 5/12 roof pitch on the top section and keep the rafter lengths the same or close.The 45 degree and 22.5 degree are also handy stops on the miter saw or chop box.
Another pair of angles that are sometimes used is a 17/12 roof pitch or a 55 degree angle on the lower roof section and 4 inch to 6 inch rise per foot roof pitch for the top roof section and to make the lower rafter length longer for more head room or a wall height or about any combination of angles and rafter lengths as needed.
Some other angles that are sometimes used are quite steeper yet 70 or 75 degree even more, on the lower roof section and about 30 or 40 degrees on the top roof section which can then gain a little more usable space in the then.. and so then just about 15 degrees or so shy of a full second floor.
I’ve used a few different combination’s of gambrel roof angles and usually try to use the 60 degree on the lower section and a 22.5 degree on the top section and make the lower steeper rafter a little longer to get a little more height as needed and and still keep the building and roof in balance and proportion.