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Add the weight of all four scales to get your total weight.
Add the weight of the front two scales for your front weight and then divide that by the total weight to get the percentage for the nose weight.
Add the weight of the left side scales for your left side weight and then divide that by the total weight to get the percentage for the left side weight.
Add the weight of the right front and left rear for your cross weight and then divide that by the total weight to get the percentage for the cross weight.
There is also a good document on the Freeman Racing site that does the math for you:
http://freemanracing.com/home/index.php/Download-document/12-Quarter-Midget-Setup-Sheet-with-Cross-Weights.html