I assume you mean the shocks are tilted forward. Tilting the shocks increases the leverage ratio so you will need to use heavier springs. Calculating the exact leverage ratio can get quite involved but since the shocks are mouned close to the axle on an XS you can roughly approximate it by the cosin of the shock angle (from vertical). Spring force = vertical force on axle / cosin of shock angle.
If you are lowering the bike then you will want to go heavier still to prevent bottoming out. My suggestion is to just get some HEAVY springs and call it done.
My bike is lowered with and extended swingarm. I bought some shocks for a Harley bagger and the thing rides like a dream. Well, as dreamy as a lowered bike can get. It no longer bottoms out, which is far more jarring than running stiffer springs.
