explody pup

I've read that stem length should be shorter for moustache bars vs. road bars. My road bike is an equivalent 69 cm (top tube + stem length) and I'm pretty comfortable on this setup so I'd like to keep my position more or less the same from one bike to the other. The bike I'm currently building up has a 60.5 cm top tube. This would leave me with an 85 mm stem, or less. I see that Nitto offers quill stems from 60 mm to 90 mm. Just wondering if anyone here has any suggestions on just how short a stem I should use. Are there any rules of thumb for moustache bars and sizing?

Thanks.