I prefer 2x or 3x going through the sump. I think the old rules were created before we had modern power heads and loop systems. The things in my sump don't really need the high flow such as calcium reactor, protein skimmer and filter bag.
Turnover rate taking head loss into consideration, or what the pump is rated for at 0'? My plan for the 150, is a dual outlet return with headloss giving me around 700gph back to the tank, which would be about 4.5x through the sump and at 0', you're talking 6x. IDK, I'm just asking about HL and the 3-5x equation.
The problem is people confuse turnover through the sump and turnover in the tank for water motion. It generally accepted that you should turn the tak over 10 to 15 x, BUT that should come from the sump, the sump should only be 3 or 4 x, the rest comes from power heads or a closed loop.