This is a question we get asked all the time. There is no exact answer or formula to altitude jetting Sidedraft or Downdraft Weber carbs. Here are the steps you need to take in order to Re-Jet your carb for High Altitude operation. This applies to both Single and Dual Sidedraft Carbs and Single and Dual Downdraft Carbs. Bear in mind that there is nothing you can do to stop the power loss you experience at high altitude. That is just a function of the lack of Air to mix with fuel for combustion. Jetting the carb properly to get the correct fuel mixture is the key to maximizing the available power.

Step-1: Find out what is in the carb now ! Do not guess and think that you have the jetting that supposedly came from the Weber factory. Find out what Main Jets, Air Jets and Idle jets you have in your carb. If it is a progressive carb make a note of what what jets were in the Primary and what jets were in the secondary. The Primary is the barrel that opens first. If you have Sidedrafts or Downdrafts with removable venturis then find out what size venturis are in your carb(s).
This is CRITICAL. You cannot tune a carb without knowing what you are starting with. 

Idle Jets: You will normally need to drop 1 Idle jet size (Meaning a step of 5. 50 to 45) for every 2000 feet above 3000 feet of altitude. This assumes the car ran properly at seal level. If you have No Sea Level Reference then you just have to get a range of jets smaller than your starting point.

Main Jets: You will normally need to drop 1 main jet size (Meaning a step of 5. 150 to 145) for every 1000 feet above 3000 feet of altitude. This assumes the car ran properly at seal level. If you have No Sea Level Reference then you just have to get a range of jets smaller than your starting point.

Air Jets: You will normally need to Increase 2 Air jet sizes (Meaning a step of 10. 150 to 160) for every 1000 feet above 3000 feet of altitude. This assumes the car ran properly at seal level. If you have No Sea Level Reference then you just have to get a range of jets Larger than your starting point.

Flat Levels anf Fuel Pressure: No changes to these for High Altitude operation. 

There is no shortcut or magic formulas. You just have to work through it. The guidelines above are starting points based on correct seal level operation. You will just need to invest in a range of jets to properly tune the engine at the altitude you are operatin at.