I personally chose to convert the entire booklet to ring-bound. The standard three-ring punch in America has holes 4 1/4” inches apart, which means the center of each hole should be 7/8″ from their closest edge. Even though the 400 series paper is decently thick, the punch should make quick work of them.
You should have two 4×6” pieces of chipboard from cutting the backing in two (if you have just one because you’re keeping the glue bound side, just use the other half of the front cover). While the punch handled the paper with ease, it probably can’t handle chipboard. What it can do, is dent the chipboard enough to mark it, allowing you use those marks as a guide so you can drill holes through them.
Otherwise, you can make two marks, roughly 3/8″ from the top edge, and 7/8″ from either side, and drill those. Double check that these measurements correspond with your own paper punch though, each one is a little different.