-- I think plant-based "everything" is likely better and more effective for humans than animal based.
-- I agree with Water (always 'take it for what it's worth' when ANYone is selling anything). But I think we're pretty safe here, because Christopher didn't start "selling product commercially" until late in his life, so it's likely he thought the Herbal Calcium formula (made from herbs) assimilated and had better energy (hence efficacy) than dissolved egg shells if he switched. To my knowledge, he didn't sell things people could easily buy or make (I could be wrong there - one of my 'hunts' is to find an original 'product list' from BEFORE his children took over).
-- I agree ("happy chickens") - if we're going to make this, it is IMPERATIVE to use ONLY the shells of eggs from free-range, organically fed, humanely treated chickens. I would not waste the vinegar on (nor would I EVER ingest) "calcium" from the eggs of tortured & poisoned chickens.
Btw, this doesn't mean the label says "cage free" or "certified organic", it means you are SURE the chickens are free-range and spend their days roaming through the grass. Cage-free doesn't indicate their feet ever touch the earth and neither does organic :(