3/16" to 1/4" will provide plenty "wiggle room" and just enough in case there's an "uh oh" in the lurch. Since it is a practice piece, if things don't turn out so good at least you'll know what not to do on the next one. A project such as this is so much fun and can be quite educational. I have a feeling you'll do just fine with it.

Hank

Yes, I did write a book. It's called "The Classic English Double Barrel"