Spraying Tums on the plant is a silly myth. It's food grade calcium carbonate, same as egg shells or limestone, without the sulfate and other impurities. Yes it has calcium but also carbonate, a salt of carbonic acid, and higher land plants have no transporters for bicarbonate (HCO3−) or need for it. (humans and ocean plants do.)

So the carbonate dries on the leaves and like dust on house plants, it blocks the stomatal pores of the plant and can't absorb carbon dioxide or expel oxygen.

If you want to make your own sprary change the source of calcium to calcium chloride or calcium nitrate. Chloride and nitrate are also salts but the plant can transport and use them.

You can crush those tums and add to the soil for the calcium but most causes of BER is from irregular watering especially in container grown tomatoes and a drip irrigation system is the best cure for that.