Easy mounting brake calliper on a Lefty
With most of them the trick is you only unfasten the adapter to get the wheel off, not the caliper itself. The adapter doesn't have any range of adjustment in how it mounts to the fork, so it can be removed and re-installed without disrupting the adjustment.

There may be a post mount only Lefty out there. If so, the difficulty you're having is basically unavoidable.

It's also important to get a pad spacer in there when the wheel is off.

Edit in reply to comment:

So, you have the XTR brakes, I believe BR-M975, that represent one of very few disc brakes ever made that connect directly to IS tabs without any kind of adapter. There aren't many brakes like this and dealing with this design involves some steps that won't be covered in generic instructions about adjusting disc brakes. This design can only be adjusted for alignment by adding and taking away micro shims between the caliper and the fork. It's a little cruel and unusual that you wound up with the combination of this brake on a Lefty, because it creates hoops to jump through that would be troublesome for anyone. The good news is that once you get the shim configuration figured out that results in good alignment, you can keep the shims there on the bolts while the wheel is off and it shouldn't require readjustment when you put the wheel back on.

Such shims can be ordered online or bought from any good shop. There are two types; one (Shimano) is open and has two prongs, the other is like a really thin (0.1mm) washer. You want the washer type because the other one will want to fall off whenever you go to remove the wheel. With the washer type, just screw the caliper mounting bolts back in to the fork a few turns once the wheel is off, down to the loctite.

The goal of the design is that with fresh pads, properly faced tabs, and everything to spec as per the IS standard, the caliper can be bolted to the frame and everything is perfectly aligned without any adjustment at all. This may or may not be achievable on any given fork or hub, depending on exact adherence to the IS spec in terms of lateral position of the IS tabs and the rotor mounting surface on the hub.