It's a good idea to get most of your calcium from food sources and only supplement when you need it. Probably best to minimize the high oxalate foods and not rely on them as a primary source (like spinach) but you don't necessarily need to exclude them unless you're prone to kidney stones. If supplementing, your body can only absorb a maximum of 500 mg of calcium at a time so it's best to take it in smaller doses. I usually take a max of 330 at a time with the hope that I'm not flooding my body with too much calcium which can potentially end up in my arteries instead of my bones.