That old Holley equation is fine for a basically stock grocery getter. The method will allow you to choose an adequate carb for your application.
But, for those with a street/high performance engine it is nothing more than a good starting
point for choosing a carb for optimum/peak
performance. There are many variables that must be considered.
Here's a good article by Barry Grant:
And if someone where trying to choose a DP style carb, Holley has a chart for that application all by itself:
If you take the cfm result you get from the old Holley equation and then use the DP chart (for same application), it's easy to see there is quite a range in the recommended carb cfm.
The 600 will work fine on the 305. Would a 500 work better? Maybe....but that doesn't mean you can't use
the 600. Especially since Edelbrock secondary activation is determined by the engines need (read air-flow).