I'd like advice on frame sizeing for the GT Tachyon. I'm 5ft 11.5in (1815mm). my in-seam leg is 820mm and arm length is 1760mm. So I have a long trunk but shorter legs. I can see that the top tube length on the GT is shorter than usual in ratio to the seat tube length. I will be using the bike for on road/fitness. What's the best size for me?