Basically, just tight enough to seal and it doesn't crush the gasket too much so cheap arses like myself can re-use the gasket
I've even been known to do them up to just 0.8 kg.m on my racer as I'd have the gearbox out 3 or 4 times a day in testing so I didn't want to be spending money on gaskets. Never had issues. No damaged threads and rarely broken gaskets.
Make sure you grease the gasket up first so that it seals but doesn't stick to the cases. It'll be easier to remove in the future, and you 'might' be able to use it again.
Then again, do that and I won't sell any more gaskets, so what you really want to do is to fit it dry, and then really tighten up the bolts with a bit of pipe on the end of the spanner. Sorted.
Strip the threads while you're at it and I can sell you some new cases as well.