check out that head loss calculator.With the mag 5, If the closed loop is small enough, and you have tons of flow, a ball valve will help adjusting it. Every time I asked for ideas, people always said more flow is always better. with that mag 18? you will probably take up most of the room in that 10 gal. you could think about splitting the out-put from the 18 and have two returns, which you could the control with the ball valve, assuming the 18 will buck like a hoarse with a burr under its sadle. Just for curiousitys sake, why dont you wanna split the 18?