I am with Mattew on this one. The reason why it is recommended to open the throttle all the way is that it is the only way you can be absolutely positive that you are passed the detent. Many experienced 412 pilots just open the throttle a tad on the right side of the detent and wait for the lightoff to prevent for getting stuck on the wrong side of the detent at the wrong time.
Note: I personally don't wait for the proverbial 12% to introduce gas. I do it as soon as I see positive oil pressure on the gage. It will happen always above 12% and it is a more critical start-no start decision item.