I found the best recipe for rearset positioning was to gobike shopping with a tape measure, lol looking at a super sport or a sport touring and taking a pic or two with a few measurements will help you set things up, and also give you some confidence behind your decision
BTW take a soft cloth measureing tape so the sales staff wont be crapping there pants with you messing about with brand new bikes. If they ask just say im comparing riding position to a (whatever bike pops into your head)