I still get my knickers in a twist over the best time to take tablets. 

My understanding is that calcium can upset some stomachs so is better taken with food.

Vitamin D3 which is often present in prescription Calcium tablets (Calichew D3) is a fat soluble vitamin and needs to be taken with food that has sufficient fat content,  to produce bile, for maximum absorption.   Magnesium is very alkaline (try putting one in water and testing the pH after a while!) and  will  have a neutralising affect on the stomach acid.  So if taken with a meal, it  will interfere with stomach digestion and prevent some of the neutrients being absorbed.  

For this reason I always take my chelated magnesium about 1 hr after my calcium/vit D3/ meal, (or any meal)

If you take vit K2, this is also a fat soluble vitamin which requires bile production to be absorbed.  But you cannot take it with calcium, as like strontium,  they compete for absorption.  As a vegetarian with a relatively low fat intake,  I tend to take my K2 with something like advocado which has a high fat content. 

Lynne H