Stable memory speed with 1600af

Highest stable speed isn't something that can be easily determined. You could just enable DOCP/XMP in your BIOS which will automatically set your memory to its recommended XMP configuration like C0v3rt said, and if it's stable you can leave it there. The motherboard you have does support up to 3600 speed memory, so you should be fine to turn it on. 

However, the only way to get the true "best" performance out of your memory is if you're wanting to manually overclock it. There's no easy way to determine what the max speed would be however, as it depends on your memory, memory controller on the CPU, and your motherboard, and you have to tweak things repeatedly. Manually overclocking memory is a very long process and not for the faint of heart, so to be honest, I'd probably just stick with enabling DOCP/XMP and call it a day.