Thanks for answers,
Ok, there was a mistake copying, it was:
Tcal = INT(1000000 / ((TIMER - Tcal)*1000))
1 mega FOR/NEXT loops divided by the time in mS.
I previously tried in a good old 466DX in Dos6.2
and the result was similar.
I'm trying now with DO/LOOP,
Tcal = TIMER
i = 0
DO
i = i + 1...