@tyssniffen
When I was using a cooling tank the set-up was much like yours. My tank was a bit higher. The bottom fitting on the cooling tank was at least a foot above the bottom fitting on the engine. As a rule of thumb, about 2.6 column of water will give you approximately 1 psi of water pressure. Your cooling system would actually be a bit (just a bit) more efficient with the tank a bit higher. That said, if the circulation is working OK as it is, leave it alone! You are in the ball park.
Eventually, I replaced the 30 gallon cooling tank with a cast iron radiator, but that was for other reasons......less anti-freeze and a faster warm-up without using a thermostat.
Cheers
Hugh