One thought on re-reading; wouldn't lowering the height of one side of the desk by a whole inch be noticeable? I'd think it easier to hide a half or quarter that and still get the desired effect. On the other hand, if you could do it and properly hide it with a glamour, that'd have possiblities, too. You ahve to love the effects you can get with low-friction surface coatings.
Author's response
Actually, no.
The normal water runoff slope on a regular floor is a quarter inch to one foot. (or one inch in four feet.) It's barely noticeable.
Now,he average office desk is five feet by three, and I cannot see Snape having any such mundane style desk. He'd want something a bit more ornate.
As for the legs, each section, would shrink only a few millimeters, so unless one was already aware of the likely cause, one would tend to miss it entirely.
Grins.