such as type of leather (nubuck alligator for me 95% of the time), length, width, stitching, type of buckle (tang or deployant), curved or straight end connection (this is where you would state it is for a FPJ piece but much better to give them the OEM strap to measure and copy to be sure), etc...
As others have already stated, Jean Rousseau is the OEM strap maker for FPJ timepieces so ordering from them would give you a perfect fit each and every time since they already know the measures.
The reason why I choose ABP over JR is because ABP provides literally thousands of possibilities whereas Jean Rousseau is more classic. JR does provide some really cool options as well but it honestly pails in comparison to ABP.
A bespoke strap involves much more than people think but the results are stunning once done correctly as it truly is personalized for you in every way.
A Tom Ford, Kiton, whatever suit maker you choose makes some unbelievably amazing suits BUT having them made for you from scratch where you choose every option along the way from A - Z makes for a truly bespoke experience and more interesting for me since I was part of the process.
Hope that makes sense πππ