i found it really easy to check, don t you?
Yes, I do, and I've done over 10 miles a minute and agree your Mach approximation is fine, , and after over 35 years of flying I reckon using the 5 and 3 times table works fine as well.......FWIW I'm a great believer in KISS, especially when teaching newbies, so I'm not sure I'd bring the tan function into it.
BTW do you really live near/in Toulouse? - 'cos if you do we're probably neighbours.