Mike,
I've run hundreds of rolls of 220 film (mostly Ilford FP4, now Tri-X 320) through my 612 Horseman RFH.
I ran a junk roll of 220 through the holder, marked each frame and noted where the counter should be. I also designed a paper label which I glued under the advance lever which let me accurately put 13 frames on a roll with no overlap...
You'll need to figure out a new starting mark and skip the first frame (it becomes the "leader"). After #6, the amount to advance each frame gradually lengthens (as the spool of film gets smaller). That's where that's label comes in, there are
marks for each frame beyond #6...
John is correct; pressure plates are designed to hold the film flat against the machined surfaces at the back of the camera. Those machined surfaces determine the plane of focus, not the thickness of the film or it's backing. That old wives tale never seems to die...
Loosing 220 film is a serious loss for me...
Reinhold
www.classicBWphoto.com