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Jeff:
Marble dust (Calcuim Carbonate) is only one of several additives you can add to give the board texture - things like silica sand (ashtray sand) are good too and give the board quite a bit of tooth.
You can find marble dust online at Pearl, Daniel Smith and other places.
I use it throughout because the addition of marble dust to acrylic gesso makes a kind of hydra-stone - it is very hard. I like to scratch and carve into my board sometimes for things like highlights and hair so I use it throughout because plain gesso in the underlayers would not hold up as well and I would be afraid of scratching through to the board surface.
As to formula, I go by the golden mean - 1 part marble dust to 1.6 parts gesso. It can be a bit of a pain to mix, so do it in a largish bucket and get a paint mixing hook for your electric drill. Mix up a big batch and put it into a bucket. Always use professional grade gesso.
Also, I don't like the Fredericks dust - it is too fine for my taste.
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