I just use a simple $5 thread counter. You don't really need an adjustable loupe. Here's why:
Think about how a large format camera works. You move the front (or rear) standard back and forth across the rails to focus the image onto the ground glass. The closer the image is to the taking lens, the further you have to separate the back of the lens from the ground glass to maintain focus. You adjust the plane of focus by changing the distance.
Well the same principle applies to the loupe. So if you place a loupe on your ground glass to inspect the image, and it's out of focus, you can easily move your eye closer or further from the loupe to get the ground glass image back into focus. Now consider how your eyes work. They can change focus on their own using the muscles to reshape your eye. That's how you can focus on a sheet of paper in front of you, and on the moon millions of miles away. So you don't even have to maintain your head at a perfect spot for this to work! You eye will naturally compensate, so long as you're within a comfortable viewing distance. And you won't even notice it's doing it.
Where an adjustable loupe comes in handy is if you're placing your eye directly on the loupe. Like in the instance where it has an eyepiece that blocks out stray light. That allows you to focus on the ground glass by adjusting the loupe without moving your eye. However, I find these totally unnecessary because I always compose my shots using a dark cloth. And the dark cloth does a good enough job on it's own of blocking out the stray light.
One of the advantages to the thread counter is it folds up small, so you can easily place it in your pocket. Also, they're cheap, so you don't have to be too concerned about scratching, breaking, or losing it. And if you ever do, they can be found at all kinds of craft and hobby stores, so replacing one isn't an issue. You could even buy a couple and save money and space over a fancy adjustable loupe. They come in different magnifications too, so you can choose the one that's right for you.
But that's not to say that you won't enjoy an adjustable loupe more. Some people do. I'm just saying that it's not a necessity to making sharp photographs.