Same as in your intialization. total your readings (the array should be int, iirc), calculate the average as a double and apply your scalar.

Your final line after the sum should look like:

return ((double)total/(double)INPUT_AVG_COUNT) * (ANALOG_TO_VOLTAGE*PRESSURE_SENSITIVITY)-PRESSURE_INTERCEPT;

Please ensure that your #defines for ANALOG_TO_VOLTAGE and PRESSURE_SENSITIVITY are either floats or doubles. Notice that both the total and INPUT_AVG_COUNT were cast to doubles. Your previous code would perform integer division on "total", probably causing the odd readings we were observing.