http://www.opticampus.com/tools/vertical.php

HELP! I am coming up with different results with computing vertical imbalance for distance the old fashioned way. Here is my sample:

OD -9.00 +3.25 x150
OS -7.25 +3.50 x180
Add 2.75 OU

Vertical Displacement = 10mm

When I compute this, I compute the power of the OD lens @ 90deg to be a cyl of 2.44 (60deg shift in axis is 75% of power of cyl) which makes the power -6.56. In the OS I compute the power to be -3.75. This makes the imbalance 2.81 base down OD.

However, when I feed these numbers into Opticampus it tells me the vertical imbalance is 3.09 base down OD.

What am I doing wrong? The only thing I can think is that the Opticampus formula is computing a different cyl power 60 deg away on the right eye axis.