Sanddrag,
Any chance you can get your hands on a reamer of the correct size? If so, the initial downward cut shouldn't make a difference. If not, I suggest that you try dropping the z in increments. You might be surprised at the results if your speeds and feeds are correct and your tool is in good shape.
Also, I would check out the HAAS website for their operator manual. That might have information on what you want. The other website to check (including their Q&A section under "Resources") is
www.cncci.com. The guy who does most of this site has written several books on CNC programming, including the issue of parametrics.
indieFan