Depends on what the ingredient is in the powdered shock. (I just noticed that you have a plaster pool and didn't list a calcium hardness reading in your post above.) If the shock powder is dichlor, then no, I wouldn't use it. It has stabilizer in it and your CYA level is already high enough. If it is cal-hypo, you might be able to use it but can't comment on that unless we know your CH reading.

You should shock until you can go from sundown one evening until within 2 hours of sunrise the next day without losing more than 1ppm of cl, and have no greater than 0.5CC. While you are shocking, test a couple times per day and each time take the cl back up to 25pm. (See the Best Guess Chlorine Chart in my signature below for the connection between CYA and chlorine levels.)