I'm installing a WS Classic in the next week or so. I'm going to run a 90 elbow right off the back - a 36" section of stove pipe up and then out through the wall. I saw somewhere a recommended 3/4 in ch to 1 inch rise on the run from the straight pipe out through the wall. Do you guys think that is enough for an 18 inch section going in to the thimble? Just looking at some pics here, seems like most have a more substantial rise than that.