How high to flood your rockwool

ive read alot on how high to flood my table with my 6x6 cubes and they say to flood all the way to the roots but no matter how much i adjust the drainage it still only comes half way up the rockwool is this ok ??? by the way these are young clones they have only been showing roots for a week out of the plugs

Rockwool as a medium has great ability to 'wick' up water/nutrients. For a 6x6 block a few inches up is all you need and the medium will wick up the water. Most people flood their grow trays for about 10-15 minutes.