Brennan,
Kent salt may be slightly low in Mg, but it is more likely your corals have been using it, along with Ca, to grow, giving you such low Mg levels.
I have never used epsom salts to raise Mg as it can seriously mess with the ionic balance of a system. Don't ask me to explain ionic balance, because I'm absolutely cr*ppy with chemistry details. That's why I use the chemistry calculator and read all those articles, half of which I don't really understand
I would suggest using a commercial product or finding the chemistry article on ALL the DIY additives and go from there. Personally, I think it's just easier to use a commercial Mg product than try to figure out the DIY stuff.
JMHO.