This is the way I set mine - going by the bottom edges of the side rails my cutter is very slightly higher in the rear, supported fully by the tailwheel, when I have my 3 pt set to the height I like to cut at (roughly 4"). The tailwheel and the 3 pt arms take the full load of the cutter - the toplink is loose and right at the middle of the 3" slot on the cutter. This slot is what lets the cutter flex up & down on rises & dips, letting the toplink slide back and forth which lets the tailwheel follow the ground. My hydraulic toplink is really handy to adjust this setting on the fly for the severe dips or hills. When cutting extremely tall vegetation I find it's better to kick the rear up more for better discharge - I mostly just kick it up with my toplink instead of adjusting the tailwheel unless it's a large job of tall stuff. If you have a manual toplink it's best to get on level ground and set the deck by eye as described above, leaving the toplink in the center of whatever mechanism you have for the flexing (not all have the slot like mine). Some guys use a chain instead of a toplink to allow the flex. You have to be careful that your flex doesn't go too far either way as to bind or let the pto shaft hit the cutter. I find that with the deck adjusted as I described, I can adjust the height of cut with the 3 pt well within the ranges I like to cut. If it's real dry I'll leave it a little higher cut, maybe as much as 6", sometimes I want a real close cut and can get one with my cutter but you sacrifice some scalping in certain areas to get it. If your land is reasonably flat, you can get as good a cut with your rotary as with a lawn tractor !! Almost forgot - you do need to level from side to side too, one of your 3 pt arm links should be adjustable. Get the deck the same height off the ground side to side using your levelling link. As drawpins bend slightly (or severely !!) sometimes, you need to re-level this adjustment occassionally.
