I imagine this has already been thought up before but I didn't find anything browsing a few pages back and wasn't sure what to search for.

Tethering war machine guess ranges

When using a war machine or any ranged attack that requires a distance guess to the target, if the guess is further then 6" past the rear of the first target in line of sight the guessed range is shortened to exactly 6" past the rear of the first target in line of sight. This cannot be used to shorten the effective range of an attack to less then any minimum firing distance stated for that attack.