Start with surfaced trim - add foam (just) below the water line to get it level. Once you have set surfaced trim , flood your ballast tank - you want your boat to just settle on the bottom (add more lead if it doesn't). Now add foam (just enough to bring the boat to neutral bouyancy) high up under to the top hull. Once you have found out how much foam you need, move it backwards and forwards under the top hull, till you find the spot where the sub is level submerged.
Remember with type of ballst tank, water sloshing about or pooling in one end when the sub is tipped will upset your trim -though a little forward motion (with a pitch controller) should setlte things out again.