I believe that in a snippets file you sent me is a measure of 25pdr equivalents to achieve levels one and two. One could then extrapolate to three and four. The degree of cover would have to form the other axis of a matrix. I have never heard of artillery bombardment reducing an entrenched force to collapse, and when you look at the first world war race to the parapet, even those enormous bombardments were not enough to cause more than a game turn of neutralisation. So my guess is that proper trenches require an order of magnitude greater than out in the open, and overhead cover might require an order of magnitude more.