If the airplane for which you're making a dipstick is a tricycle, be sure that the nose is not unusually high or low while you're calibrating, it can make quite a difference in the dipstick reading if the oleo is depressed, or over inflated. But yes, making your own is a good way to do it. I like to wire a ring to the end of sufficient diameter that were I to drop the dipstick into the tank, the ring would prevent it falling all the way in.