If it stays up on you at the tightest position then I wouldn't worry about it. That is, in fact, how I prefer to wear my kilts because it allows for the most overlap, and allows for some growth if it happens to occur. My waist size is anywhere between a 32-33.5. I order 34" waists from Stillwater when I go to buy a kilt. I have to wear it on the tightest position, and it works great.

Look at it this way, if it stays up, then it fits. If you feel it is a tad loose for your liking, get a belt on around it to help hold it into place.