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Are yo working with a lennox, carrier or what? Indoor wet bulb is and excellent starting point. What is the Outdoor ambient? What does the charging chart on the inside of the electrical cover say it should be based on outside temp? What is the indoor dry bulb? If the indoor dry bulb is below 70 degrees, your suction pressure is going to be low. 10 degrees subcooling could be a good rule of thumb on new equipment but you should refer to the install manual to be sure.