no2 requires a bigger gap because of high temps in the cyl. This causes the ring to grow more. .004" per in of bore is pretty much a standard spec. So .014"-.016" would be great. And .016-.018 in a nitrous motor.
Be sure to get all the burrs of the ring after you file it. You also want to cut the ring from the outside edge toward the center so as not to chip or round of that outer edge that will go against the cyl wall.