The general rule of thumb is to use 1/8" size up with a .058 wall thickness from the tube you're trying to sleeve. For example if you are making a seat lug with a 28.6 seat and top tube you need to buy a .058 walled 1 1/4" tube to slip over the tubes. .058 is prety thick so it takes some work to get them closer to the acceptable lug dimensions roughly 1mm in thickness. If you have access to a lathe that can greatly reduce time spent on sanding buy turning down the O.D. of the tube.