One thing you may want to do is to get a fresh part of the cable under the pinch bolt. You'll need to shorten the housing somewhere to compensate and hopefully you have enough to do that. Or just get a new cable and route it through the current housing.

I'll have to say though in all my years of doing bike stuff I've never had a cable that slipped and I just tighten by feel. With brakes I'll apply more torque because you certainly don't want those to slip. I've often wondered why brakes don't have a secure routing that's more fail-safe, not that I've ever had an issue.