Regarding having one flat corner. I’m not nearly as experienced as some but here’s what I came up with and it seems to help. I got a piece of cord long enough, with knotted loops, to reach across the diagonal of the shelter. I then tied a simple loop at the ‘end’, a second at the length of the short side, a third the length of the long side, and the final the length of the diagonal. this cord serves the dual purpose of a top loop tie out for my MLD duomid for inclement weather
When I’m picking a spot for a pitch and laying it out I drop stakes at the 4 corners, adjust as necessary on both lengths and then the diagonals which must match to get the layout ‘square” (4 90 degree corners and not trapezoidal), then set/stake the 4 corners, the pole and presto.
Saves time fiddling. In fact it helps things go pretty fast and also you can discover if the space is too tight before getting too far along. The cord weighs a few grams and has a useful purpose as a top guyline if your shelter has a top loop.
You may also need to tweak your ‘measuring’ cord loops a little to adjust for the position of the stakes in the shelter’s loops (the ends will be longer with the stake loops than the length of the fabric sides) or for a higher pitch, higher side levels in gentle weather but since you should be starting from a true rectangle this should be relatively easy.
I think that sometimes you get some variation in the level of the footprint and even if the part of it you plan to sleep on is level this can throw off the appearance of the pitch a little, flatten out one corner. Even with 4 90 degree corners the shelter can be a little off because the bottom plane is tilted. 2 or 3 inches over 115 or so may seem really level for sleeping but it’ll throw off the ‘balance’ of your pitch.