If you're draining old fork oil and adding new stuff, you may not get all the old oil out. Then your refill amount is going to be what you measured out plus what's left in there. In effect, you'll end up with more oil in there than you actually measured out and poured in. So, the most accurate way to set fork oil amounts is to slightly over-fill then suck out the extra to a certain level measured from the top of the tubes, springs removed, forks fully compressed. A general rule of thumb is to set the oil level 6" from the top of the tube.
I just did this on my '83 forks. I initially filled them with 7 ounces of oil. They were a bit stiff so I decided to remove some oil and hopefully lessen the increased air spring effect more oil adds. Checking the level on what was in there, I found it at about 5 1/4" from the top. I set it to 6" from the top. I put the oil I extracted into a measuring bottle and found I had taken just under an ounce out combined for both tubes, so maybe about 3/8 of an ounce from each. These '83 forks are a bit of an experiment for me. Instead of replacing the springs, I just added about 1" preload spacers. This made them a bit too stiff so I shortened the spacers to 13/16" and pulled some fork oil out. They're better now.
Here's my thinking and the theory behind all my spacer work - the stock adjustable top caps add 10mm of preload for each step, so a total of 20mm can be had. I've tried stock springs like that and they were still too soft. So, I wanted to preload them just a bit more than that max available setting. I figured 1" (about 25mm) would be good. But, what I failed to take into consideration was that you add another washer with the preload spacer, and that adds another couple of MM to the preload. So, I had ended up with 27 or 28mm of added preload, almost another full step of adjustment had there been one, and that proved too stiff. So now, with my 13/16" spacer AND the added washer, I've got maybe 23mm of added preload, and that seems pretty good.