This helped a lot, thank you!
First draft for Golden Sun Dark Dawn 16:9 Widescreen:
It looks like the formula is (16/9)*4096=7281.77777778 (roughly 7282) and then 7282 in hex is 0x1C72 (ie. 00001C72?)
Just leaving the math here so other hobbyists can turn 21:9 into 9557.333 (ie. 00002555) or...