If I understand you question correcly, all you need is a calculator. I have never done this, but it should work this way:
1) Calculate the number of measures per minute.
2) Multiply that number by the time signature. ( X 4 for 4/4 time, X 3 for 3/4 time) = beats per minute (tempo)
Starkeeper
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I play Roland EM20 and Yamaha PSR550