85lc, may I suggest Wright's book? He took several loads, duplex and straight BP, to both Kynoch and Birmingham Proof House and had them proofed. The pressure readings of the duplex loads were less than or equal to the BP pressures. Until I perused his results I too was leery of duplexing. 10% is Wrights recommended maximum for the smokeless kicker, IIRC and, I've been able to use as little as 5% with desired, satisfactory results. 7% has been the maximum I've needed to get the results I wanted. I've always looked for loads that would regulate and not pursuing high velocity.