Can I use dietary calcium supplements to solve end blossom rot on tomatoes and squash. If so how much. I've been watering properly?

I wouldn't/ Calcium supplements are usually provided in the form of calcium carbonate, which is most insoluble in water and which plants cannot absorb (it reacts with out stomach acid and becomes calcium chloride when we eat it). Although it will eventually turn into more soluble forms, with the action of soil bacteria, this takes far too long to solve blossom end rot.
More importantly, plants need quite a lot and using supplements would be hideously expensive. Buying calcium sources for plants is much cheaper.
The usual way of increasing soil calcium long-term is to add powder gypsum or limestone in the winter. This gradually breaks down and releases calcium over the summer.
If you need to increase it quickly, a liquid plant food that says on the back that it contains calcium is you best bet.
Remember, though, that although blossom end rot is caused directly by lack of calcium, the usual reason is actually insufficient or erratic watering. When the roots are dry, the plant cannot absorb calcium from the soil properly. You say you are watering properly, but are you really? Tomatoes in pots need watering daily, twice a day in hot weather. And even in the ground need a good soak at least every other day....Show more