What size ND filter to get?

[shannaford]

Hi all - reaching out for your collective wisdom once again, this time on a B + W 110 3.0 ND filter. My question is not so much about the filter (as I've pretty much decided this is the best option for a 9-10 stop ND that I'm after) but more as to the best size to get.
At present I have a 75mm Rodenstock lens with 58mm front thread and a 150mm rodenstock with 49mm front thread.
So, given concerns about vignetting on the wide angle lens etc should I just buy a 58mm filter and get a step ring for fitting it to the 150mm lens?
I know, I know, this may not fit any future lenses, and I may be looking at a telephoto when finances allow. But here's my cunning plan - is it possible/sensible to mount this filter to the back of a lens if it doesn't fit on the front element? I am currently borrowing a 360mm Tele xenar, and it looks like it would be no prob fitting this filter to the back of the lens with a step ring.
Should I perhaps be buying an even larger filter (72mm? suggestions?) as future insurance and using a step-up ring on the 75mm lens as well?
At first I thought something like the Lee or Hitech system would be the obvious answer, but they don't make ND filters this dark, which means I'd be stacking them, and I don't really want to do that.
I've been doing my head in going over all the possible combinations/possible problems with each and figure I'm not the first person to confront this one - be very grateful for your experiences/thoughts.
With thanks,
Scott.

[Bjorn Nilsson]

Mounting filters on the back of the lens is no problem. It's actually promoted on Sinar cameras, at least those with a dedicated Sinar shutter. The pros of doing this is that there is one less piece of glass in the front to take care of or to shade off.
Else, you seem to have taken all possible variations into consideration. There is nothing really wrong in your discussion (with yourself) above. It seems like only time will tell when and what next lens you will purchase. The idea of getting a larger filter and some step-down rings sounds like a good idea. You don't say if you use any kind of sun shade, but if so, you have to take that into your calculation too.

//Björn