Which brand of tube? rocsim has all the dimensions, and if you dont have them, I could get em for ya. Just need to know what brand.
Making a fin marking guide is easy enough. Simply take a piece of paper, and wrap it around the tube. Then cut it so the ends are right next to eachother. Then take the lenth of the sheet of paper (after it is trimmed to exactly one wrap around the tube), and divide it by the number of fins you want. Then take that number (it should be inches or centimeters), and put a mark every, say, 3.3". It wont be the same for your tube, but for my Staples 4" tube, it was somethin like 3.343". It will be different with yours, I am positive. Say the lenth of the piece of paper was 9", and you want 3 fins. you put a mark every 3". Then you wrap the piece of paper around the tube again, tape it, and vola'! a tube marking guide. Thats just how I do mine. There are other ways. I would reccomend using a calculator