When I used IO it wasn't that low, are you mixing the salt before you scoop it out? Sometimes the different components can settle out in shipping and storage. So if all the mg was at the top and youre reaching the bottom of the bucket it can run low. I think there's something up with your batch. Has this been an issue for more than one bucket of salt?
I get reef crystals now cause it's close in price on amazon w/ free shipping (yay Prime!). I haven't checked the mg on a new batch, but I haven't had any trouble keeping my tank above 1200.