About as much as I know is that the amount of calcium you retain is governed by the cortisol levels anyway.  So taking high doses of calcium would do more harm than good for sure.  Thats because what ever your body cannot retain due to high cortisol levels, it will have to be excreted and thus is a strain on the kidneys and a causatory factor of kidney stones. 

The best way to actually increase your ability to absorb calcium in conjunction with taking in calcium rich foods such as green vegies and skimmed milk, is to increase muscle mass and the hormones that help calcium to be absorbed, such as dhea.  Thats why weight training and exercise help really.  They serve to do two things.  One: lower cortisol levels if they are too high and Two: cause anabolic levels heighten and help with calcium uptake.  So as long as your getting plenty of exercise and eating the right types of foods, you’ll be fine.  About the only supplement I take is tums actually which have calcium carbonate in and I take those with meals. Just don’t go nuts that’s all and take mega doses of the stuff.  I feel ten times stronger than I did this time last year with doing just those few things.  My calcium levels would have been low at that point.  Calcium absorption enhances fat loss also.