Hmm. Recipe 1, 3A calls for 5 cups of MgCl per gallon of mix. 2.5 cups of Mg solution will supplement 6 gallons (6.4 actually) each of the Alk and Ca solutions. Without knowing how many pounds of Mg flake equals a cup, you can't know how much Mg flake you'll need. My guess is that their 4 pound bags equals 2 1/2 or 5 cups, since they've apparently repackaged it to accomodate the Holmes-Farley recipe.
I suggest using the "contact us" feature on the Buckeye web page to ask the question. $12 for 8 pounds seems like price gouging to me though. It's just ice melt. You may be able to get it locally in Oct/Nov. You can probably find the manufacturer's web page, contact them, and see who sells it in CO.
I found a Denver source on Thomasnet:
B & B Blending, Inc.
5285 Fox St., Unit B
Denver, CO 80216
1-800-875-6320