Planning a shoot. Want to shoot a telephoto shot at a specific location. Going to take test photos of location with an 80mm, and then I want to crop into the image in photoshop such that I would get the view of a 135mm, 200mm, 300mm, etc. so that I can determine exactly what lens I need to rent. Shooting primes, zooms not an option. Everytime I search I get spammed with information about the sensor crop factor but I'm looking for the formula that determines how focal length effects image magnification. Is it just a linear transformation? 200mm lens gives 2x magnification of 100mm?..

Thanks!