A complete overhaul and re-development of Gary Grigsby's War in the East, with a focus on improvements to historical accuracy, realism, user interface and AI.
relates to the discussion in 20.4.3 where you had enough artillery to trigger a drop in the fortification level. Usually this is triggered by combat engineers (as that is easier to achieve).
The actual value needed for artillery is not in the manual - usually this is related to either a desire not to reveal the formula or that the formula is insanely complex. In this case I think the latter as its not just the amount of artillery but the type while for infantry and engineers its a fairly linear relationship between what you commit to the attack and the effect they have.
I maybe wrong and the formula is in there somewhere, but if so I can't find it back [;)]