Best and easiest way to get a good speed reading (for me) is to draw level with the target and match speed until it is steady at 90 or 270 degrees in the scope,less time messing around with complex mathematics in a combat situation
Ideally you want to be observing the target for 15 - 30 mins to get a good steady result.
I personally find making adjustments 'on the fly' and using my own judgement and observation to be more satisfying than performing ridiculous mathematical equations.The MK I eyeball will take some beating yet.