Yeah..it's not a good idea to go with the volume of oil idea. That means you have to have the SAME about of oil in the forks to start with..and that isn't likely unless you had them completely apart for sleeves, seals, valves, etc.
That's why the 'oil level' method works better. Set that by using a syringe with a piece of tubing attached to extend to the desired level.
I use a 100mm (4 inches) level, but.... to each his own.
I just changed the oil in my forks. No fancy tools handy so I took a piece of tubing about the same diamater as the carb vent tubes and cinched a tie wrap 120mm up. Then i siphoned the excess from the forks back in to the bottle. I had the tubing laying around the garage so cost was nothing.
huge syringes can be had at a feed store that carries horse supplies. They are only a couple bucks each. I got a syringe, piece of flexible tubing and a length of clear plastic rigid tubing. Ya take a block of wood, drill a hole that the rigid tube fits snuggly in, attatch the flexible tube and syringe to it. You have a great tool for about three bucks.