Hi
I'm Swedish and have he same dilemma with the Imperial vs. metric conversions.
If you buy both the engine and spinner nut from Tower hobbies, you don't need to know the meassurment in metric, just buy the 1-4/28 spinner nut if that's th same meassurment as the prop shaft of your engine. (You can find many engine manuals online on the manufacturers websites)

Something I must advice you to buy if you'll ever build a kit is the "Hobbico builders ruler".
This thing is the most useful builder tool for us 'non-imperial' kit builders building a US made kit.

Just pick up a dowel or wood part and testfit it in the ruler, until it fits a slot, all slots are marked with the Imperial meassurment, perfect!