Hi, Steve,
I've never tried to use a torque wrench on my 90 cylinder base nuts - there's not really enough room to get in there under the bottom cooling fin with anything more than an open-ended spanner, particularly the front RH side one that's tucked in against the exhaust stub. I just use one-handed firm pressure on a normal 10mm open-ender.
If you want a torque figure, I'd suggest around 5 or 6 ft.lb. It's a steel nut going onto a steel stud - pretty much the same as the head nut arrangement.
Hope that helps,
Graham