Hi, I have just finished building a 72" Yak and with all Yaks they tend to be tail heavy,due to the short nose.
To find the measurement for the gap between the spinner backplate and the front lip of the cowl,you must first find the correct CG.
With all your flying equipment in place just move the engine along the mounts until it balances slightly nose heavy at the manufacturers recommended measurement.
Then slide your cowl on to suit and leave whatever size gap you like,because as you will find out the gap is not as important as compared to getting the plane to balance correctly.
You will most probably have to place your receiver and flight batteries up the front and possibly even have to add some stick on wheel weights to the engine mounts to get it to balance.
But dont dispair Yak's do fly nice.
Oh to cut nice neat holes try using a Dremmel tool with the round sanding drums on them.If you dont have one you can Drill a hole first and the wrap some sand paper around a piece of small pipe and put some CA glue on it,and use that.
Best of luck..