Raising CYA with Dichlor - need formula please

[BigDave]

Dichlor should add about 9ppm CYA for every 10ppm FC. 10lbs of dichlor should add about 15ppm CYA to a 40,000 gallon pool.

[Mm_putnam]

Awesome, thanks BigDave for the info.
Leads to a couple other questions:
1. If I want to get to ~70 CYA (using 70 as my goal with my new SWG - unless someone says I'm off there) I'll need ~50 lbs. of dichlor, sound right?
2. My plan was to use up what I had left from last year's dichlor to get it stabilized but if I'm right above I'll need to buy more. I'm OK doing that or can I buy straight up Cyanuric Acid and use that instead of dichlor for the stabilizer and go ahead and get the salt system running for the chlorine maintenance?

[chem geek]

You can certainly use Dichlor in your pool to simultaneously add chlorine and raise the CYA level though obviously you can't raise the CYA that quickly since you don't want your FC to get too high. Nevertheless, to use up what you've got by adding it over time is fine. Then if you still need more CYA you can just buy pure CYA from your pool store, as you wrote.