I am spec'ing out a Parlee and had a similar question. I know what my XY coordinates are (pedal, seat, bars). once you have the right top tube length, head tube is the next controlling factor. Optimally you don't want more than 2 to 2.5 cm of spacers about the head tube. In my case I went with the longer head tube and -6 on the stem. I didn't care about look, but it will be horizontal.

The downside of my setup is that I only have 1cm of spacers so I won't be able to get the bar much lower. But I am already running a pretty big drop so I think it works. At least that's what Parlee reco'd!