Well there aren’t too many options for water soluble calcium. I think there’s mostly just cal nitrate, calcium chloride and calcium EDTA.
I’m already using calcium nitrate, which is great for getting the bulk of the calcium needed. The issue is that later in flower, plants continue to be hungry for calcium but their need for nitrogen starts to drop off and they want more potassium. However, potassium is also an antagonist toward calcium and magnesium. So you really need to keep the calcium at the ~2:1 ratio with potassium, while reducing the nitrogen.
If you add more calcium nitrate, you also increase the nitrogen so that doesn’t work. That’s where supplementing in calcium EDTA helps. It allows you to reduce the calcium nitrate you use, yet keeps the calcium level up.
I’ve also experimented with calcium gluconate, which could actually be even better than EDTA. It’s chelated with amino acids that the plants can also use; however it’s a lot more expensive, lower density and lower solubility. Calcium chloride you can use in very limited amounts. Adding chlorine isn’t something that’s generally desirable.
Edit: There’s also calcium citrate, which might be worth experimenting with. It’s probably cheaper than EDTA, but I do wonder if it would affect the pH. And also calcium carbonate – which I don’t think is any good for hydroponic applications.