I don't know how many ounces to use, I've never taken my EX500R apart and measured anything on the bike. You do like FOG says, measure the inches down from the top that the oil level is. Either use the top of the fork spring or the top of the fork or anything else that won't be changing it's position. I actually use a hollow brass tube attached to the end of a brake vacuum pump to set my oil level. I mark the distance from the bottom end of the tube with a magic marker. I then fill the fork with a little bit too much oil and then just drop the tube into the fork until the magic marker mark lines up with the top of the spring and suck out the excess oil. Do this on both forks and your set.
Be carefull though because unless you know how much fluid is in the forks to begin with, you don't know if you have the right amount specified by Kawasaki. What I do with a new bike is take the fork caps off and measure each fork for it's oil level. Write those numbers down. Then I drain all of the fluid out of the forks and refill them with the amount of ounces specified by the manufacturer. Then I measure the fork oil level again and write that number down. If the new fork oil level is different than what it was I just try it to see if I like it. If the fork dives too much while braking, I will add oil, about 1" at a time, until they work better. If I've got too much oil in the forks they will become much too stiff and I remove about 1" at a time until they are better.
Changing the fork oil level is more for street driving, it's not the answer for road racing where you are driving on a smooth road and don't need a soft ride and are trying to prevent the bike from bottoming due to G forces around corners. No road course is as rough as the street is, with it's potholes, tree parts, car parts and a few parts from old Harleys and Triumphs, etc., etc., etc.. When you drive on a road race course or speed through your favorite canyon road, you put up with a harsher ride than when you're just putting around town. So you increase the metal spring rate. But I just drive around town and there don't seem to be many places here in North Las Vegas to go around fast corners (also, the roads are very, very sandy out here and are very slippery), so I prefere a soft ride and I'm not going to change the metal spring rate.
One note about cutting a spring that has a flat top on it. That flat top is to give the spacer lots of area to press against and just cutting the spring will leave a pointed end in contact with the spacer. If you can, you should heat the end of the spring till it's red hot and flatten it out so that the spacer will get more area to push on. I know that nobodies going to do this, I just though I might as well tell you the right way to finish the end of a spring, just so you'd know. Cutting the spring is just the cheep way to stiffen it up, saves you the money of buying new springs. I've done it many times on cars and bikes over the last 40 years.
When you cut a spring the new spring rate will be the old spring's length divided by the new spring's length times the old spring rate. So if your old spring was 20" long and you cut off 2" and it's old spring rate was 30 lb/inch (about .55 kg/mm) it's new spring rate will be:
20 / 18 x 30 = 33.3 lb/inch (about .6 kg/mm)
If you go to Racetech and buy springs for an EX500R it will have about 47 lbs/inch spring rate (which is about .85 kg/mm). Probably good for racing and canyon carving.
If you go to Provressive Suspension you'll get a 30/45 lbs/inch (.54 to .80 kg/mm) progressive spring rate. Sounds good for the street to me.
A note about changing the oil level being like milling the head - it's not. Cutting the spring is like milling the head because once you've done it, you're done. You can never go back, you can't add metal back onto the cut part. Changing the oil level is no big deal, if you don't like it you just put the oil level back to where it was and everything is the same. It's simple, cheep, repeatable, AND UNDUABLE.