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John,
Before starting the engine, turn the screws in until they seat closed. Open each the same number of turns, about two. Start the engine and let it completely warm up. Reset the desired idle speed. Turn one screw in at a time until either the vacuum or engine speed decreases, then back out about 1/4 turn. Repeat with the other screw, then reset idle speed as necessary. You can adjust for rpm by ear, tach or by vacuum as you said. These screws only effect the mixture at idle and will change with the weather conditions anyway.
Before starting the engine, turn the screws in until they seat closed. Open each the same number of turns, about two. Start the engine and let it completely warm up. Reset the desired idle speed. Turn one screw in at a time until either the vacuum or engine speed decreases, then back out about 1/4 turn. Repeat with the other screw, then reset idle speed as necessary. You can adjust for rpm by ear, tach or by vacuum as you said. These screws only effect the mixture at idle and will change with the weather conditions anyway.