The WillTravel design height of the camera bodies is based on the minimum Flange Focus Distance (FFD) for that particular focal length. FFD in this context is the distance from the back of the shutter and to the filmplane/Groundglass. And then adapting for longer FFDs by extending the Frontcap length. Different lens design / brands in the same focal will have slight differences in FFD, thus check the table below to get the Frontcap and funnel which matches your lens.
The design criteria’s is to be able get the camera to infinity focus i.e. minimum extension and at the same time be able to focus nearer by turning the frontcap further out, whilst have a sufficiently engagement on the threads to ensure stability and light sealing.
8 mm thread pitch is selected for focal lengths of 90mm and less. Focal lengths from 100mm has a thread pitch of 12mm. This allows for a focus change from infinity to quite closely (1-2m) with one turn of the frontcap.
The internal funnel is there to prevent the frontcap with your valuable lens from suddenly disengagement on the threads and thus the risks of falling on to the ground and be damaged.
Below are what I currently have found of lens data, particularly FFD data. Should your lens be missing from the table, send me a email (morten@kolve.org) and let’s research. Or even better should you have data that could be added.
Maximum calculated Frontcap length to be able to reach infinity, i.e minimum extended. A few millimeter shorter might be ok, key is to still have good thread engagement when you are your maximum close up focus. Note this is NOT a macro camera design ?.
Minimum calculated Funnel length to be able to turn Frontcap one full turn from infinity. Longer might be possible, but you need to test for mechanical vignetting. (With a film exposure, groundglass peeping is not sufficient.I have learnt this in real life. ?)
Sources:
http://homepages.tig.com.au/~cbird/nikkor/sw.html#SW90
http://www.kenko-pi.co.jp/horseman/e/lens_list.html