Davo, the ideal height is where the grain bag is a safe distance from the metal kettle bottom where the heat transfer to the metal is intense enough to melt the bag material. The distance should be minimized. PP wrote recently (with diagrams as I recall) that you do not want a significant volume of liquid trapped between the bottom of the bag and the bottom of the kettle. The temperature in that space will probably be very different from the middle of the mash. If one applies heat to restore a target mash temperature, there had better be efficient stirring of the mash or portions of it will have very different temperature exposures. If the "trapped" volume is not stirred well, then temperatures will be uneven especially if your kettle has a tri-clad (thick) bottom. The smaller the trapped volume, the better.

If you want a number, my guess would be 1 cm. Half of that could also work, and 2 cm. starts to be too high. Pressure cookers come with a separate metal perforated disc with an edge that would keep the bag off the brew kettle bottom. Vegetable steamers have ~1 cm legs to hold up a perforated circle. Cooling racks for baking are the right height, but mind he gaps as a sagging bag could touch the kettle bottom.

It is advisable to stir the BIAB mash if adding heat. Getting the added heat distributed from the liquid touching the bottom throughout the mash is the goal.

I hope this helps.