It's actually far simpler than any of the answers posted so far! You don't need trigonometry, or field of view calculators at all, all you need is multiplication and division!

Firstly (all else being equal) the size of your object in the image is directly proportional to the focal length (if you double the focal length you double the size).

So if you know the subject distance for your 85mm lens, you can work out the subject distance for a 135mm lens as follows:

new subject distance = (135/85) x old subject distance

For the second case (known subject height), we can exploit the fact that the triangle formed between object and the lens aperture is similar to the triangle formed between the lens aperture and sensor. Thus the laws of similar triangles can be used to find the missing side (which represents the subject distance). In other words the size of your subject on the sensor divided by the focal length is the same as the size in real life divided by the subject-camera distance.

So if you know the subject height and the sensor height (between 15mm and 16mm for most crop sensor DSLRs in landscape orientation) then you can work out the correct distance as follows:

distance = (real height x focal length) / sensor height

All units must match, so if you use the focal length in millimetres, then the subject height, sensor height and distance must all be millimetres.