Accept my reply not as advice but more so as a complete novice still learning and what tokens I've personally found underneath a few crashes.
My first feeling controlling roll was my back foot WAS the foil...made sense to me b/c I had my back foot out of the back strap and right above the foil. If my foil tested my patience and went toeside-ish I felt my back foot doing the same so I applied back foot heel side pressure. This all happened on my 3rd sesh when I put in solid foiling tacks. Whatever roll angle my back foot was at the board/foil replicated.
My 5th sesh, I put the back foot in the back strap and hence the foil is more between my feet now so I 'think' I'm using a blend of both feet and body to adapt. I think you will go thru a few variations of what's working. And, BTW if you think roll is a problem tacking upwind....what till you do a mile downwinder
On a side note, in 7 sessions, I'm loosing some of the feels I had in my first 4 sessions and I can't really articulate even to myself what is causing what now. All I know is there is so much going on and the brain is in crazy overdrive it's like the conscious just shuts down and the whole experience is sub-conscious and that is really freaking cool.
It's like the moment you start thinking about technique, you look up and your wingtip is a foot from the water and you're like holy shit! So the brain just shuts off and says 'dude, just take it in and pick up the pieces later.' I didn't think I'd get so hooked on foiling, but I think I'm really going to enjoy this. The exploration potential and massive boosting potential is exciting.
Oh and btw I'm seeing that like Peter said on the downwind tacks my roll is much more correctable with yaw movements fyi