Are you purchasing anhydrous magnesium sulphate? If you are dehydrating it yourself, why not just skip that bit entirely and add the heptahydrate to a smaller amount of water? The correct volume is easily calculated although I haven't bothered to do any of that yet.

A probable reason why this technique seems to work is that the magnesium will be displacing the alkaloid from its tannates. This speeds up the extraction process - hurrah!

IMO the anhydrous magnesium sulphate bit is a waste of time and effort not to mention the fuel bills if you're roasting your own. Just use it straight from the tub as heptahydrate and use less water. Where I live, pharmacies and health food stores sell massive tubs of the stuff, pharma grade, for not very much money, presumably you know this already (except, hopefully, where I live  )