Knowing what shocks are on the thing, like a part number, C-C at ride height, etc, would be helpful.

There are no 2.5" ID, 11" springs for a common coil over that I'm aware of. What are the diameters of the springs?

The answer is - typically, you want the lightest spring that will hold the shocks at the desired C-C at ride height without coil binding before the shock is bottomed out, or you run out of threads. If it wants to separate too violently at the hit, control it with the shock. Information on Length At Coil Bind for springs is available from the manufacturer.

I always end up with lighter springs than what the math tells me.

A short spring will have to be heavier than a longer spring to fit in the same space and do the same job. Simply because the long spring will have to be compressed more. Compressing the spring more stores more energy. That helps get the separation at the hit.