Thank you all for responding,
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Henrik you are right ... I asked * / 1251?

George RPM monitoring is continuous, therefore I can not do it at compile time.

They all seem to be correct but the easiest way for me is the response of Alain.

If possible I would like Alain explain to me how I get to that equation.

I'll do some tests

Excuse me for my bad english
Best regards

Marcos.