First, I would be suspect about the reading from your pool shop. It would be unusual for your pool builder to put in a system with an SWG and not add any CYA to the pool, or even to have any pool outside exposed to sunlight without any CYA. Is your pool exposed to the sun at all?

If your pool wasn't exposed to sunlight and if it was not an SWG system, then you technically wouldn't need CYA though for rather obscure reasons I would still probably recommend a small amount of it. However, SWG systems require CYA to be run efficiently, even if there were no sunlight. CYA not only combines with chlorine to form a chemical that is more resistent to breakdown from sunlight, but the CYA also makes an SWG cell more efficient by combining with the generated chlorine so that too much chlorine doesn't build up in the salt cell near the plates.

I suggest getting your own test kit that includes CYA (either K-2006 from Taylor or get Ben's test kit or other kits recommended on this forum). You can also ask the builder if he added CYA. If he did, then it will still be there since it does not break down (at least not quickly).

If you truly do not have CYA, then perhaps that is why you have to run your SWG at 45% because some users of this forum only have to run their SWG at a much lower rate. Running your SWG at higher power levels and for longer periods of time will wear the cell faster so you will have to have it replaced sooner (and that's expensive). In this case, I would suggest you adding CYA, but again I would get your own test kit since I would not trust the pool stores.

Richard