[Clockunder]

As a rough guide :

If the length you want to get onto the portrait orientation on a full frame (or 35mm film) is x metres, then the required distance with a 35mm lens (actually should be 36mm) is x metres (this has to do with the magnification factor. The magnification factor is distance to optical centre divided by the focal length used. So the image of a 1.8m tall person standing at 1.8m away will be 1.8m/36mm = 1/50 times the original = 36mm tall which fits in exactly on the longer side of the 36mmx24mm film/full frame image sensor).

If you are using a y mm lens, then mulitiply the distance by y/35
and
If yours is a 1.6x crop factor camera, then muliply by another 1.6
and
If you are on landscape frame orientation instead of portrait, then muliply by another 1.5 (3:2 aspect ratio).

Finally, add 1 more focal length + another 10cm and that will be the approximate working distance.

For e.g. if the person is about 1.8m tall and you want to take a half-body portrait of about 0.9m :

If you're using a 35mm lens, then the distance is :
a) if on full frame, then 0.9m + 0.035m + 0.1m = 1.035m

b) if on 1.6x crop factor, then 0.9x1.6 +0.035 + 0.10 = 1.575m

c) if on 1.6x crop factor and shoot landscape framing, then 0.9x1.6x1.5 + 0.035 + 0.10 = about 2.3m

If you're using a 50mm lens, then the distance is :
d) if on full frame, 0.9x50/35 + 0.05 + 0.10 = 1.44m

e) if on 1.6x crop factor, then 0.9x(50/35)x(1.6) + 0.05 + 0.10 = 2.2m

and so on.

For simplicity, you may just ignore the addition of the last 2 terms (+ 1xfocal length + 10cm) and just add about 15cm.

Working distance is one issue to think about and DOF is another to consider carefully in deciding which lens to use. For portrait, usually 50mm or 85mm is good for DOF.