[kalee20]

Earthing to a transformer fixing tab screw is better than nothing. It's the sort of thing that has been done many times before, and the world has not ended yet.

However, today's standards (which even for home use by folks like us can be regarded as good practice to abide by) says that an earth fixing must be independent of a mechanical fixing. This is so that the earth fixing has the minimum load on it, so it is less likely to come loose with vibration. And the cable must of course be slack, not tight (the official term is a 'strain relief loop').

So, you couldn't argue for instance that the left-hand bolt is the (single) transformer fixing bolt, and the right-hand one is the earth - because it clearly has the transformer hanging off it as well. If the left-hand one works loose, the weight of the transformer hangs purely on the RH one so it will soon come loose and you lose your earth, just at the time when the loose transformer has its tags touch the metalwork.

Personally, I would drill a discreet extra hole and fit a countersunk screw for a solder tag (or crimp tag) and nut.