It unfortunately doesn't work that way since the recommended shock level is for a rough rate of killing the algae -- not a precise thing. However, I'll take a stab at this since you did see things get slightly lighter green at 25 ppm FC so let's say this is perhaps double the rate of just keeping up with algae growth and let's say that's around an FC of 5% of the CYA level. So if 25 ppm FC were about 10% of the CYA level, that would be a CYA of 250 ppm. This is, of course, a total guess -- it could be 150 ppm or it could be 300 ppm. Even if it were 100 ppm CYA, then a half drain would not be bad and given their offer of free liquid CYA (probably "Instant Pool Water Conditioner") you don't have much to lose.
As Jason wrote, you unfortunately can't rely on the CYA test very much when the water is murky with algae. Nevertheless, if you can take the pool water by itself (i.e. don't add any reagent to it) and clearly see the black dot filling up all the way to the top of the tube, then that's only going to affect the CYA test if it had actual low values (so would read higher). If it reads 100 ppm then the murkiness won't affect that very much unless it was REALLY murky (and since you can see a couple of feet, that may not be enough to affect already high readings on the CYA test).
If you want, you can do a test in a bucket of pool water to see the effects of chlorine and how much it takes to kill the algae (you can do this while doing the half drain/refill). If you have a 2 gallon bucket (preferably white or light-colored), then every teaspoon of 6% bleach adds 40 ppm FC. Look at
this thread and notice that after 10 minutes of shocking the pool gets noticeably better with clearing of much of what is in the water leaving more solids on the bottom to deal with (of course, this situation is on pool opening). So you can see how much chlorine it takes to find a noticeable difference in the bucket which you should keep outside exposed to sunlight so that the algae grows just as it may still be doing in the pool.
Also, you should really, really get your own good test kit such as the TF100 from tftestkits.com
here. Though this pool store you are using sounds more decent than most (they seem to have some understanding of the chlorine/CYA relationship), you're going to need your own good test kit in the future so should just get that now. In the meantime, keep the FC level up in the pool using chlorinating liquid or 6% unscented bleach if you can, just so you keep the algae somewhat at bay. If you wanted to use Cal-Hypo for shocking, you could do that if your Calcium Hardness (CH) isn't too high -- I suspect it's low due to the drain/refill depending on the CH of your fill water.
Though you said your pool was 24,000 gallons, you didn't say whether it was plaster, vinyl or fiberglass. Also, do you have a cartridge filter? If so, then that would explain the high CYA level since continued use of Trichlor without frequent backwashing can raise the CYA level quite quickly. Adding 2 ppm FC per day would result in an increase of 36 ppm CYA per month or over 200 ppm in just a single 6-month swim season. Evaporation and refill doesn't affect this at all since evaporation concentrates and refill dilutes right back to where you started, though adds to the pool whatever is in the fill water (typically TA and CH).
Richard