I'm sorry you've had this problem, & I hope your dog will recover.
I've actually never had a problem with the dials on the dewormer tubes. I haven't used Ivercare specifically (I don't think it's even available here) but I have used the various other ivermectin dewormers. I had a near miss once, I gave a dose of paste to one of the foals--she got the right amount; the ring did stop the plunger at the right moment, then it gave way. I'd quit pushing the plunger by then, so no extra paste got pushed out. I figured it was my own fault--I always double check/double tighten those rings before I give the paste, & that one time I didn't do my double check.
Even though I usually "can't afford" the actual cat & dog dewormer from the vet, that's what I buy anyway. I use Drontal tablets for both--all need doing right now & it's going to cost me $75 for the cats & I'm not sure what for the dogs (2) but regardless, it's a simple effective dewormer & so easy to pop down the correct number of pills, no chance of an overdose--I figure it's cheaper to buy the proper dewormer than to have to fork out a huge amount of money to fix a problem such as you're having now.