I'm getting ready to cut up my basement slab for a bathroom. I've got the pipes laid out and vented for shower, toilet, and sink and re-route of the kitchen drain above. I'll be trenching about 10 feet to reach main drain. Any hints on setting the slope for this run and all the 2 inch going in it? I know it's 1/4 inch per foot. I figure you get the slope right for the long 3inch run and then set the slope for 2 inch runs off that? Nervous about getting that slope right when glueing! Thanks