There is literally only two things you need to know about metric:
One, the prefixes: Metric prefix - Wikipedia
Two, it ALWAYS goes in powers of 10. 1 meter is 10 decimeters (not used much) is 100 centimeters is 1000 milimeters. 1 meter is 0.1 decameter is 0.01 hectometer is 0.001 kilometer.
Or for an even simpler way of seeing it, you can easily go by 1000’s and not miss anything; 1000 millimeters is 1 meter is 0.001 kilometer.
As far as getting your scale right… Last I checked, 1 Unreal Unit was “fixed” to be 1cm. Which means that 100UUs are 1m.
With these numbers, your scales will be as follows:
Inch: 0.3937 scale.
Foot: 0.03280 scale.
Yard: 0.01093 scale.
Mile (why are you using mile?): 0.000006214 scale.
Then again - if you don’t plan on using assets made by other people, the scale shouldn’t matter - since you can just set yourself up to be internally consistent.
But yeah - personally I’d just learn to use metric, a lot more useful in a lot more ways - since virtually all 3D software run natively on increments of 10, and it’s a lot more simple and consistent system - 10’s instead of… What is it again? 12 inch to the foot, 3 feet to the yard, 5280 feet to the mile (or 1720 yards to the mile) or something like that?
Much easier to remember 10’s or 1000’s.