A good beginning target is 35-40. You can always add more if you find that you need it, but you can't get rid of it if you go too high except by draining. You can use trichlor because they do contain CYA but it will take some time before it builds up enough to help you. If it were my pool, I'd do one of 3 things.

1. Just buy some CYA and add some separately to get to around 40. (Probably about 11lbs.) You can add it slowly to the skimmer while the pump is running and then keep the pump running 24/7 and don't backwash for at least 4 or 5 days to give it time to dissolve. Also, don't test for CYA for at least a week or else you'll just be wasting your reagent as it can take that long to dissolve.

2. Add enough CYA separately to get to about 20 and then use trichlor pucks for awhile and monitor CYA. When you get to 50, stop using the pucks.

3. Use dichlor powder for awhile and monitor the CYA and stop when you get near 40. (Dichlor will put the CYA in faster than Trichlor. For every 10ppm of cl that it adds, it will also add 9ppm of CYA. With trichlor, it adds 6ppm of CYA per 10ppm of cl added. But, dichlor dissolves much faster than trichlor does so it wouldn't take as long for you to get a CYA reading than if you just use pucks.)

Any of the 3 ways will work but with any of the methods you choose to do, you'll have to add chlorine frequently in the first week or so. Until you get enough CYA dissolved and into the pool and can register a reading, you'll lose chlorine quickly to the sun.

I will caution you that dichlor and trichlor are both very acidic so if you use either of these products, monitor your pH.