Black Bottom Tomatoes

I noticed that a few of my tomatoes had black bottoms. By my understanding this is from inconsistent watering where the calcium is not reaching the fruit (My grow solution may also be too weak for the plants at this stage in the growth cycle). There are other factors that may cause this problem but I have been a little irregular with my watering so it seems a likely cause. Why have I been irregular with my watering? I collect my water from a nearby spring a short distance travel by car but sometimes the plants draw the solution up really fast. There were a couple of occasions when the plants went without water for a day or two at a time. I thought that I may have been over watering them so I was not really too concerned if they had occaision to dry out. Obviously this was incorrect so I will chalk it down as a lesson learn't. The affected fruit is generally higher up on the stem which supports that the calcium is not reaching the fruit  Any affected fruit should be removed as there is not way to save it.