Selecting item from drop list based on calculation result & excluding 0 or null values in average

 In this automatic culvert size calculator, I would like the correct culvert size to be automatically selected from the heading culvert sizes dropdown list and the checkbox array opposite the culvert discharge area: ie. in the image the checkbox opposite the 800 culvert should be selected and 800 should be selected from the drop-down list. In the image of the form, the Aavg field was calculated using the 'average' calculation function, however, I would prefer if the value was the mean of the values that are greater than 0 or null, so as an error isn't caused by a failure to enter a full six measurements.