I don't want to open a can of worms here but, I know the formula. It works out great and should be closely followed with a double-pumper carb. But with a vacuum secondary you can go about 100 plus cfm more without hurting bottom end and it enhances top end performance. Look at the vacuum secondary as a variable cfm carb because unlike a DP, the vacuum secondary is only going to use what it needs. The 750 still has a 1.4 main venturi size which is the right size for a 350-400 sbc.
If you dynoed that combo with a 600 dp and a 750 vacuum secondary the 750 would rule because when you go to wot on the 600 you open all 4 barrel instantly killing all vacuum to the engine down low then as it comes back it runs out of air flow up top. The 750 vacuum only opens the front barrels then lets the engine open the back barrels as needed, there fore up in the top end you have enough cfm to make more power. Oversizing carb has all to do with venturi size and booster signal. A 850 holley has a 1.5 venturi which really hurts booster signa on smaller engines. JUST MY OPION AND EXPERIENCE