3/16" is the general measurement. Install the cowl loose, then install your spinner and prop. Tape or CA three pieces of 3/16" balsa to the back of your spinner, they're gonna be our standoffs to get the distance exact. Shove the cowl up against the three pieces of balsa, which will give you an even backspacing all the way around. Then you can tape your cowl in place against the fuse, and drill your holes. Once you get the holes drilled, just pull the standoffs off the spiner.
I"m not familiar with that airplane, so I don't know if it has dedicated cowl mounting blocks on the firewall or if you have to just drill thru the sides of the fuselage. If it's the fuselage, leave your fuel tank OUT! when you drill and install the screws for the first time. If you're just going thru balsa sheeting, you'll want to go behind the holes and glue some hardwood blocks there for the screws to thread into. If you already have hardwood mounting blocks on the firewall, no problem. One other thing you can do, if your cowl is a loose fit on the fuselage, I.E. you have 1/8 to 1/4 inch between the cowl and the fuselage when the cowl is mounted properley, make some standoffs out of hardwood and use them for your mounting points. It will also give you some extra air exit space.

PM me if you have any other questions or want some pics...

Andy