How high should I set the flood level? (flood depth)

For most e&f setups, the small planters are used with a grow rocks medium, usually combined with a small (1-1/2") rockwool minicube.

The flood height for this combo, should be to a height just touching the very bottom of the minicube, but no higher than 1/4 to 3/8" past the bottom edge of the minicube(s).