Tablets can vary quite a lot these days especially with all the 4-in-1 type products. But here's how to look at it -
For every 10ppm FC added by trichlor, you'll add 6ppm CYA.
A typical 99% pure 3" trichlor tablet is roughly 8oz of trichlor.
So, if you stick those numbers in pool math, you can get a rough estimate of the number of tablets it takes to get 10ppm FC.