For a 210 mm that would be 8 1/4 inches at infinity. If you were to ever do any closer focusing then you will want some additional rail length.

I shoot a 450 on my 8X10 for close up work. That would be the equivalent of roughly 225 on a 4X5. I find myself fairly regularly having my bellows extension out to 26 inches when 18 inches is infinity for the lens.

So if I were you and think that by some small chance that I may want to do a close up shot of something at some future date then I would allow something on the order of 60% addtional rail length above the 8 1/4 inches that the 210 mm lens would take at infinity. That would be about 12 or 13 inches total rail length.

However it is your camera and you should do what you want.