You can, but you don't know how much got left behind or if you left behind the same amount in both forks. Being a little over or under is not that big a deal but different amounts can be a big deal.
You can make one. I assume it is saying fill it until it x mm from the top. Just get a coat hanger, straighten it out and tape some aquarium hose to it and measure out the distance.
Add oil as you BLOW on the tube. When you hear bubbles you got it.