(Pretty funny that hendi_alex and I were writing at the exact same time earlier today.)
Agreed, that M. Bartholomew can go pretty far "out there" at times, but his spacing recommendations for, say, carrots
worked well for me. I was careful to put my zucchini
at the edge of the box so they had space to trail over the side and crawl as they wished.
OTOH, his spacings for beans
(either bush or pole) and chard
are (shall we say?) quite optimistic! I handwrote into my book "2 to 3" under chard, where he had recommended 4 per square foot. I don't know about y'all, but my chard get pretty big. 4 per square? No. We harvested one entire plant one time rather than just the outer leaves, and the remaining 3 were much happier. I think we went with 6 bush beans per square vs. his "8 to 9."
We made these changes simply by watching
our plants. I had gardened in the '80s and through summer 1998, when the after-effects of a car accident in December 1995 finally became too severe to ignore any longer. When I returned to gardening in 2008, SFG had become all the rage (probably due to the re-release of the book in 2005), so I checked it out.
If I had enough space, I'd follow Jeavons 100%. But I've squeezed only 96 sq.ft. of raised boxes out of our "yard," and have had to give up on the lovely 32 sq.ft. of rented space I had the past 2 or 3 years off-premises.
So my approach now is a combo of the two intensive advocates' approaches.