Jan may be right that a 2-cup dose would be ok, but I think I'd only do a cup of Borax to start with. Test it again after a couple of hours and then redose if needed. You have an awfully small volume pool and it won't take much of anything to see a pretty significant change in numbers.

If you want to use the pucks, do so until you get your CYA to 50. If you find that you are losing chlorine too fast (because of not enough CYA in the water yet), then you can always supplement with a little bleach.