Did the pro shop use the right size nock for your string? There are different sizes to accommodate strings and serving material combinations. Did they double nock? I thought about stretch out of the gate but this one seems to be stretching way too far for all the more the bow was shot. 3/4 of an inch in 50 draws... wow, either you ended up with the worst Zebra string material that came off the roll as "seconds" or they forgot half the strands at the factory. Ok, we all know the latter isn't the case, but there is an easy way to nail down a problematic string suspect. Take an upper and lower tiller measurement now that the nock point is where it should be. Take the bow back to the bails and have a go. Recheck your tiller after your shooting session. It shouldn't change drastically. Another possibility for increased stretch out of the box is that your factory string is lacking twists. This can accelerate the rate of stretching like you describe. I would twist it up so it retains a cylindrical shape and hammer the bails again. Assuming that the limb bolts aren't backing out like mad, which I doubt, you should be good to go. I do however agree with GVDoc, in that an aftermarket string, (I prefer Winnerschoice), is a good investment. Short of seeing the bow, I'd twist it up, measure the tiller and let 'em fly. Let me know how you make out.