Glad you too like the Powerbox - good aren't they!! Initially, in the warmer, drier weather I set Neutral (lift-off) at about 350 and the Footbrake setting at 500. This was indeed fierce, but I live in a hilly part of Surrey and I just love seeing all the 'free' Amps flying into my battery as I came down the slopes under perfect control without having to use the brakes. In fact you soon learn not to need the brakes at all. Which is very good, until you remember that following cars don;t see any brakelights, so get waaay too close. In these higher value settings, I tended to 'tickle' the brake pedal just to show a brakelight to those following.
However, when making good progress (!) on a slimy, slippery road, I noticed that the lift-off regen was starting to upset the handling and I can completely believe that it would be an unsafe nightmare in the ice and snow in this setting, so backed things off somewhat. I've settled on 200 and 350 respectively, which isn't too far up from stock in practice. However, remember that a long push on Button No.1 will disable Regen completely, which is what I plan to do when I take it out in the snow, come the time. Too much regen in those conditions and you'd have to be VERY careful indeed
(EDIT: ... and then another push of No.1 switched it back on of course)