One of the good ways to figure out your magnesium-calcium needs is general personality. Too much magnesium tends to be serious, unable to take quite enough joy in activities, tending towards the more logical and somewhat depressed side of life.
Adding calcium carbonate (like from calcium pills) won't help, but calcium Bentonite does. If it's low calcium across the board, problems sleeping, problems with blood clotting, and sometimes issues remembering things should show up.
A blood ratio of calcium can be somewhat misleading because it does not give a good understanding of the balance of calcium across the whole body. There is a better method of using hair mineral analysis, which when you use a good company, will give you paper documentation with recommendations for helping your minerals come more into balance. (Aka, instructions for food and supplements)
Where to go for those papers will depend upon where you live, google searches and seeing folk's opinions on different labs will always help.
With hashimotos you MUST ALWAYS START with zink and HIGH LEVELS of healthy selenium formulations- go for Life Extention Super Selenium Complex ('bout 10 bucks on iherb.com ). NEVER use yeast-based selenium, it does damage!
I mention a specific selenium because I had to learn to search long and hard for the right ones after starting iodine. Yeast-based selenium gave me motion sickness for weeks, and damaged my eyesight a little. The complex product listed above has vitamin E in it, required for using selenium.
If you react poorly to the selenium, load up your diet with red beet root, beetroot juice, or as a silly bugger solution, Tri-methyl-glycine (TMG, cheap as dirt, jarrow formulas has a good tablet version. ). Zink and selenium help get the sulfur metabolisim going.
You'll nuke your thyroid. Hashimoto's especially is indicated in the Iodine VWT forum as being selenium-deficient in a hard way. Give it 3-5 months of selenium & zinc, with likely extra b-complex to get your mineral and glutathione reserves higher before doing the iodine.
Iodine breaks down infected and worn-out cells, and it can get the thyroid to break itself down by producing too much hydrochloric acid (what it uses to clean the blood on the way to the brain). Selenium balances out the HCL cycle, so the thyroid does not harm it's own tissues.