A cooling flush for me is 4oz then pull a 2oz shot then flush about 2oz or less for clean up. If i was going to pull a second shot i would wait 15 seconds approx and pull the second one.
I don't think your gonna gain much by lowering the temps but you will lose steam pressure for frothing. I'll measure how much I pull during a cooling flush in a few minutes. Whats your procedure to flush? I pull it down so I can start my pull for a shot immediately without a rebound. Start to finish I am done in like a minute or so.