That general rule of dividing by 3 or 4 is a good one to get you in the ballpark.
Just buy a couple of 9ft tapered leaders of the tippet size you need. That and 2 or 3 different size spools of tippet material and you'll be good to go. 3X to 5X tippet will cover you for probably 90% of all your trout fishing needs. 6X and smaller is very light tippet and wouldn't be too well-suited for use with a 6wt rod anyway.