More preload is what we're after, right? A stronger or more compressed spring will make the valve open at a higher vacuum reading.......right? This isn't a rate thing, it's a when thing. Opening from say 9.5 to 12.5 just needs a stronger or more compressed spring, correct? Might can get by with just tightening the disc a few turns. As long as the valve opens at the vacuum reading you're looking for, you've achieved the objective, right? I just don't think it's that complicated.
