You can run it a little low, you certainly wouldn't be the first person to have ever done that. You can also top it up, if you know what the "air gap" should be then this is easy - collapse the fork as much as you can and measure the distance from the top of the stanchion down to where the oil is, and fill it until you have the correct distance. If you don't know what the air gap should be then you're stuffed, maybe just put a few slugs of oil in and see how it goes or empty it completely and refill with the correct amount. My question is how do you measure down from top? Because I might be tempted to do this with non leaking fort to match with leaking fork. How do you get your little it of oil in? Use a piece of wooden dowel as a dip stick, the oil will leave a witness mark. Remove the fork caps to do this.