I would place the power heads on opposite ends of the tank attached to the rear glass. Submerge them several inches into the water and point them so that the flow makes an X pattern in the tank.
To measure how many times you turn the tank over, simply add up how many gallons per hour all your filtration equipment pumps....then divide that number by the number of gallons in your tank. ie filtration pumps 100 gph
in a 10 gal
tank= 10 times an hour.
As far as whether circulation is key to a marine tank...I think it is one of the most important and overlooked factors. If you turn your tank over 10 times an hour, you are in pretty good shape, and it is one less variable to look at if a problem occurs. HTH