I set my enlarger up to make an 8x10 from 35mm and then adjust the starting exposure to 1/2" at f22 which is the shortest cycle for my timer/enlarger and lens. This usually does the trick. Using longer than about 1/2" can cause reciprocity failure problems. In fact, the 1/2" is probably too long but it will work. I have then gone on to dupe transparencies onto negative film (B&W and color).

PE