Bear with me folks...

I know that one can work out focusing distance from the old formula;

1/p + 1/q = 1/f

where p is bellows extension, q is lens-subject distance and f is focal length. BUT, whenever I've seen that formula quoted it's always been followed by a cavea t along the lines of "for a theoretical lens of zero thickness". My question is, how important is that caveat in practical terms?

The reason I ask is that I only get 300mm of draw on my Wista bellow but I'd lik e to use a 300mm lens (the Nikkor M 300/9 is the one I have in mind). Don't want to muck about with extension tubes (needs to be a small, light and reasonably c heap solution) but I figured that if I could find a recessed board, maybe 30-35m m deep, and mount it in reverse then I'd be able to focus down to 3m or so - whi ch is ample for me. However, if that formula is way out for modern lens designs or one should be measuring from the rear-element rather than the centre, then it could be an expensive mistake!

Any words of wisdom would as always be much appreciated.