With oil height set at 120mm (below the top of the tube, fully compressed fork legs, and no springs), you might experience very harsh bottoming. At 75mm it will never bottom.
I put a set of Hyperpro progressive lowering springs on my F800GS. I removed enough oil to hit their spec., for my springs, of 110mm. The first time into the woods, it started bottoming really harshly, even on bumps that my KTM would totally ignore. Back home, I spent a Sunday inspecting and measuring forks.
The problem is that the frugal (a polite word for cheapa**) German engineers didn't include any parts for a bottoming cushion on the F800GS forks. Personally, I've never seen this in a fork before. Instead, they specified an oil height of 60mm. Adding the volume of the spring and fork cap, that would leave about 3mm of space for air when fully compressed. If you could bottom, that would compress the air space from about 250mm when extended to 3mm compressed, raising the pressure to about 83 atmospheres (bar) and over 1,200 psi. Given maybe 4 square inches of cross sectional area, you are looking at around 5,000 lbs. or 2,200 kg. to bottom the forks. Can't happen. The forks never bottom. Don't need cushions. Saves a few euros. Maybe someone got a bonus. The forks also get a lot stiffer just part way into the stroke. However, if you follow the Hyperpro spec, developed for best overall handling, and add another 50mm of air space, then you only have 5 atmospheres (bar) and 75 psi. You are only getting a few hundred pounds of resistance instead of a few thousand, so the forks now can bottom, flat metal to metal, and they do.
With a 110mm oil height, the bike handles smoother but sounds like a car crash when the forks bottom. Also rims bend easier (had to straighten one of those too). Riding just on highways and regular gravel roads, my guess is that it should rarely, if ever, bottom. However, go off on some trails with any speed and you'll hear the forks say "OUCH!" every few minutes.
How I got the 75mm: On my particular springs, I measured the volume of the stock spring and my Hyperpro spring. The Hyperpro springs have more coils and are larger, so they fill up about the same volume as 12mm of oil height. Stock spec is 60mm. Adding 12 mm for the larger springs gets to 72mm, which I rounded up to 75mm. With your 120mm height, If it doesn't bottom on you the way you use the bike, don't worry. However, if it does, start adding oil up from 120mm to 75mm. This is just my untrained amateur opinion.
Originally Posted by ride2little
I removed the fork tubes (after removing the front tire, brakes and fender).
Then I popped off the fork caps, removed the old spring (the left one in the picture) and dumped out the old fork oil.
I filled the fork tube full of 10W fork oil up to 120mm below the top of the compressed fork.
Installed the new spring (the one on the right in the picture) with the tighter winds toward the top.
Then I put the fork caps back on, slid the tubes into the triple trees and re-installed the front wheel, brakes and fender.
Today's ride told me that I did the right thing. I really like the way it feels now. Much less sag, stiffer, yet not stiff, less nose dive while braking and the front tire feels more planted. Both on and off road.