To achieve effective cooling with a proper temperature stratification of the fluid, you should make sure the fluid level is just slightly higher than the top edge of the black divider which separates the miner compartment from the fluid recovery channel. Fluid should barely overflow the divider, creating a waterfall effect in the fluid recovery channel. If you use lower miners (example S17 / S15) you can remove part of the divider lowering the needed fluid level. The correct fluid amount shown on the drawing below.