Out of curiosity, from what version of FileMaker did you upgrade?
There are two basic methods to calculating aggregate values such as averages, calculaton fields with aggregate functions and summary fields. Which did you use?
I don't have every version of FileMaker, but when I create a number field in FileMaker 5.5 and a summary field to compute the average, records with blank fields are ignored, just like FileMaker 10 and 11 do so I'm suprised this worked for you in a previous version.
A simple fix is to add a calculation field such as Reserve Field + 0 and clear the "do not calculate if all referenced fields are empty" option. Then set your summary field to take the average of this field instead of the Reserve field.
I know this is terrible but I was on version 4. So going to version 11, I new would be a challenge. The good news is that I was able to figure out a solution last night. What's even better, what I figured out is exactly what you recommended.
Thanks for your interest,