A CYA of 40 is a good reading. We typically recommend 40-50 for most pools. If you want to bump it up to 50, that would be fine.

Since you dropped below the recommended minimum of 3ppm and you have a little algae on the stairs, then you should shock your pool to kill it and clear it up. Also, brush the pool especially where you see the algae. Run your pump 24/7 while you are clearing it up.

Once you get the green cleared up, then the testing process for your pool will be this. When you test your chlorine each evening, add enough to take your chlorine level back up to around 6ppm. For reference, using a quart of 12% will add about 2.3ppm of chlorine to your pool. By the next evening, if you find that your chlorine level has dipped below 3ppm, then take it up to 7ppm. You want to take it high enough that you never drop below 3ppm by the following evening.

Hope this helps.