Hey Classic... I was stressing out over this whole deal recently myself. I purchased some redline synthetic for oil and started researching how to change it. It is possible to do it using a mityvac, and there are plenty of people who do it that way, but for me, I decided I wanted to KNOW 100% how much oil was in there when I finish the job, and I want to know that both forks are absolutely equal. So I took my entire front end apart and did it the slow way. Yes, it was a project, but with 25K on it, I feel better knowing everything is correct. I was also able to clean my caliper pistons very well, change my brake pads over to Lyndall Z+, check my steering head preload and grease the bearing, etc.
While I had it apart, I ended up changing out the stock springs with Race Tech springs and I lowered it 1 inch. To check the oil level, I took a 10" long piece of 3/8" hard plastic line, like for a semi truck brake line, and I made a mark for 6". Then I slipped the mityvac hose over the end of the hard plastic line. I stood up the fork leg on the shop floor, completely collapsed, and poured in about 10 oz of oil, cycled the fork a few times, then I stuck the plastic tube down in the fork tube to the 6" mark and used the mityvac to pump out any excess. This way anything above 6" from the top was sucked out, leaving the correct amount of oil in the fork tube.
I also added more preload as you mentioned, by using a 1.8" preload spacer instead of the 1" they recommend. To get the fork tube plug installed, I had a friend hold the tube upright with the spring and spacer and plug on top. I stood over the fork with a short piece of 2X4 and placed the block carefully on top of the fork tube plug. I then pushed straight down, applying my weight to compress the spring, at the same time he held the fork tube up and started turning it... essentially we were turning the fork tube while holding the plug stationary. It worked fantastic and we had both tubes back together in literally 15 seconds each.
I took it for a ride the next morning and it was fantastic, no more clunking pulling out of a parking lot onto the road. The ride was much smoother yet firm... just the way I like it. I wish I would have taken some photos of the process, but I didn't. I am super happy I did it the way I did and I don't have to worry about how much oil I have and I can duplicate the results or change it if needed too, if I wasn't happy with the ride.