I might be able to help, but I've only done two batches under a heat plate. I have the RentACoop brand, and they tend to run about 120F at the underside. I set the back two legs to be about back height of the tallest chick, then set the front two legs (meaning the direction they'd be entering from) a tick taller. The temperature of the bedding/pads below is typically 85-90F. All my chicks (including bantams) have seemed very comfortable, but I also make sure the room they're in is around 68-72 degrees for the first few days (on the higher end if they've been shipped).
EDIT: Should add that if your plate runs cooler than this (maybe 110-115ish), then it should DEFINITELY be at true back height all the way, or even a tick lower. This will allow the chicks to press up against it safely.