No you won't screw up anything. First off you want to find out why it's leaking oil. Which is, you guessed it, coming from the seal. Take a flat bladed screwdriver and gently pry the dust seal downwards. Clean out any gunk. Take a business card, piece of film, thin plastic etc and run a corner up into the seal all the way around the fork leg. Hopefully it was just a piece of dirt that was causing the leak and you now fixed it. Push dust seal back up and fill up the fork.
It's been awhile since I changed out the fork oil in a KTM fork, but I recall 10 wt being pretty stiff fluid. I just replaced the oil in my closed chamber Husky forks with AMSOIL 5 wt shock therapy fluid and it was brilliant afterwards.
For KTMs I've always checked the air chamber length (110mm or something like that look it up first...
). I've always done this with the forks off the bike in my hands, upright. I take an old windex bottle and throw away the bottle, measure out the air chamber length on the windex suction tube, and cut to length. Now I insert the straw into the fork tube so that the sprayer screw cap is resting on the rim of the fork tube, and "spray" into a towel or something until I can spray no more. Voila, perfect air chamber height.
Since your forks are presumably still on the bike, unless you can get the ass end in the air 27 degrees or so until the forks are straight up and down, not sure how you would measure it.
Depending on how bad they are leaking I would be inclined to leave it be for a day and fix it right when you get back. By fixing it right, again depending on hours, I would probably buy a fork rebuild kit with new bushings, seals, wipers, orings etc. Change out the fluid and really clean the insides of the forks.
Alternatively you could also unscrew the other fork leg cap, assuming it is at the correct level, and measure it with a zip tie, and match the oil level on your other forks leg.
EDIT: reread your question and I guess you have the fork out? In that case, it's easy. But it's easier to slap the fork back in the triples to loosen up the caps, kinda tough to try and hold those tubes and get the cap off at once.