I just did this on my 85 TD. We measured another wagon height, and got 25.5", asphalt to highpoint on wheel well, if I recall. Detach the linkage rod between the valve lever and the torsion bar back there, and push the lever up or down (car running) to get the height you want, reset length of linkage rod, re-attach. Mine looks high at 25.5", we think because he has 15" rims and I have 14s and more gap. So I may bring mine down a hair, maybe an inch.
I suppose if you had the thing way too high or way too low, it might have some impact on driving, but I don't think the setting is hyper-critical. I drove my wagon for months with the tail down, on one occasion loaded to the gills, and I didn't notice anything other than the weight wagging the car a bit.
Let's see if someone says I am off the mark.
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K