Good points here.
Yes, propping the strutless door as high as you can helps reduce the torque preload - but don't let the edges of the door hit the T-roof panel.
Yes, if you use an Allen key be sure it's long enough. It needs to insert fully into the bar and still have enough shank length for the bracket to fully escape from the bar onto the key shank before the bar can be unwound.
If the wrench is only partially inserted in the bar you run the risk of mushrooming the end of the splines making the bracket harder to remove or install.
How well one can lever the other (short?) leg of the key with a cheater bar I cannot guess. But it can be done.
Doing it solo likely probably is best done with a bracket that slides right off the splines like it's supposed to. With rust/scale that's less certain to be the case.
And as far as how far to move the adjustment my experience is that each spline jump is worth about 4-6in. of door rotation measured from sill to bottom of the door in the hang test. Another hint might be to look for pencil matchmarks on end of bar and bracket. The factory method calls for matchmarking before adjustment so you know where you came from. Going back to the 'factory' setting could be a good start. There are notes in this forum that you can pick up a mini-adjustment tweak by using the bracket mounting hole tolerance before final bolt tightening to put a little more or less twist into the bracket/bar for a given spline choice.