Good Afternoon, Matt,
Assuming that your enlarger has a scale along one standard, along with an indicator giving the distance of the head above the baseboard, the following formula should work well in most cases.
New Distance/Original Distance (Squared), Multiplied by Original Printing Time = New Printing Time.
Example: Original Print Exposure--5 seconds with Enlarger head at 13 inches.
Next Print with the enlarger head at 26 inches.
26/13 = 2; 2 squared = 4; 4 x 5 seconds(OT) = 20 seconds for the new larger print, assuming, of course, no change in f-stop. It's just an illustration that light falls off as the square of the distance.
This is, if I understand correctly, essentially the same idea as Donald and Lee L have described; it just uses enlarger head height instead of print length as the measurement. Since my Beseler MCR-X has a scale on the right-hand standard, it's the most convenient way for me.