This is the formula for changing exposure time for different enlarger heights for the same print:
TO*(LN/LO)^2=TN (the "*" is a multiplication sign, "^2" is "squared")
Where: TO is the original time
LN is the new length
LO is the original length
TN is the new time (at the same f-stop).
Exposure calculations are calculated using the law of inverse square.