I grew up on the Canadian border so my math is backwards from yours ..
What I did was memorize 100kmh is 62mph, 50kmh is 31mph and that took care of 90% of the driving.
In the US, if you are doing 30 in a 25mph, they usually won't bother you unless it is a School Zone.
Basic speed limits are 25, 35, 45, 55, 65 ... so if you just memorize those two .. and go a little over or a little under, you are safe.