Can't give you any specific guideline to the accuracy as i just always buy the biggest size available!
I can however give nothing but high praise for the Roadlite 6.0 i bought from them a few weeks ago. Amazing value for money (£920 delivered), lovely finishing touches, really light for the price, looks ace and rides fantastically, so much nicer than the ageing Trek 1500 it replaced (although it may be that it just fits me slightly better).
Wouldn't hesitate to recommend it.
Re: sizing, i'd pop into a local bike shop and throw your leg over a few road bikes to see what feels right. Personally i think their calculator is fine but i have read other peoples posts and some think it's way off?