Welcome to the forum
As in all things bushcrafty I do not see myself as an expert in this topic but can give you some comments.
As per Dwardo with some additional observations
I think most fire baskets are about 4 inches/10 cms off the ground and this gives an order of magnitude. So anything above this should be ok and anything significantly below it probably won't be.
However, regardless of the height of your basket above ground level, you should clear the ground below the basket - and for, say, one foot/30 centimeters around it of all debris , grass etc and put this on one side.
When you have finished with your fire basket you should make sure the ground below it is cool. This will involve actually touching the ground with your bare hands. If in doubt pour water over the warm ground but this probably won't benecessary.
Finally put back the debris/grass etc so that you leave no trace