Iv got about 25 dual plug buttons right now, The idea as I know it was to give a "even" burn to your fuel, in a .67 make sure you have the correct head button bowl for high nitro, as most will detonate the head button and piston to death with 50%+ fuels. My advice would be send it to a CMB engine guy, and have it blueprinted.

40% is what i run in my sport boats, 50% and higher for the 21's, and 25% with 18% oil for my .90~101 motors.