Thank you Kerry, Brad.
Brad, I know how to calculate this :-), but albeit Shen-Hao spec mentions the max. draw of 360mm, it can only be achieved in particular sutuation by twisting the front out of its frame and tilting it up. In fact, I have never tried it yet, but without it, I believe the bellows can reach 300-330mm, so 300mm lens apparently will work up to some close range, probably close to what you mentioned and Kerry confirms it (as he tried with Nikon 300mm).
Kerry, regarding the shutters, I would have to acquire new shutter also, not only the scale. Also, I'll have SK Grimes to fix the scale on the shutter. So in my case, the total (lens + shutter and necessary work) may easily reach 600$...