To unscrew the cup, you need less than an inch of additional height over the 22.25" total skimmer height. As such, if you leave yourself 23.5" or even slightly more, you will have enough space. If you have the opportunity now and can leave yourself an additional few inches, you might thank yourself down the line as you would allow for enough space for a self cleaning head which are great additions to these skimmers as they keep them running much more efficiently without the need to remove the collection cup regularly. I run the SM250 myself and only remove the cup every several months as my SCH keeps my neck nice and clean. It's also nice to have a bit of additional space above the skimmer so you can lift the lid and peek in at the foam. This is especially useful when you are fine tuning it. The neck will get dirty quickly so you won't be able to see what the foam in doing in the neck which is where lifting the lid comes into play.
Now for some setup advice. Hopefully you didn't choose a skimmer that' oversized for your display. People often make the mistake (with these skimmers) of sizing them based on the systems total volume and not the display size where the load is concentrated. These skimmers are really conservately rated and can handle larger sized tanks than they are rated for and if you oversize them too much, you will encounter consistency issues unless you tune it really wet. Depending on the size of your display and the amount of fish/load in your tank, you will want the skimmer in 8"-9" of water. The lower depth for a heavier load and the higher depth for a lighter load. If your Supermarin is the new one with the RD3 pump, I would suggest a power setting of 37-38 watts (I have mine at 37 watts which is perfect for my system). This assuming the skimmer isn't oversized for your display. If you have a really light load and or the skimmer is oversized, then you may need a bit more power and the higher level (9" or so) to maintain reasonable consistency. You will find that out after break in and once the skimmer catches up. I usually recommend using the sump level to get the water level in the skimmer in the correct ballpark (with the wedge pipe wide open) and then use the wedge pipe to fine tune. You are aiming for a level where the line at which point the water/bubbles turn to foam is just above the white collar where the cup connects to the body at the base of the neck. From there you would use the wedge pipe to adjust the level in the skimmer up or down to fine tune between wet and drier skim.
