Set up your camera on a tripod, with lens of interest. Attach string to the edge of the cardboard card with your format cut-out. Hold card so that it matches your camera's view with lens of interest, and bring that string to your nose or cheek or whatever uniform reference point you use - tie a knot in the string. Do that for every lens you use, and remember (or write on the edge of the card) first knot = 21mm , second knot = 35mm, etc etc. You now know how far out to hold the card for each lens. Cost: free.