I was just reading a tech article about holley carbs and for power valves they say to measure your manifold vacuum at idle and devide that by two. If you measure 15 for the vacuum, then get a 7.5 power valve. They were a little more vague on the vacuum secondary springs, but they say many people install lighter and lighter springs until they can feel the secondaries kick in. But I have read other places that you should not even feel the secondaries kick in on a properly tuned carb. What I would do with this knowledge is try out the various springs until you are feeling the secondaries kicking in and then go up one level of spring tension.
