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.