Average formula that averages the last 6 filled cells of a range

I am working on a spreadsheet for a company that has FEW Excel users (and those few consider formatting experience). I am trying to setup a 6 month average worksheet where I can write an Average formula and select a 15 month range to cover. Then as the user enters values in new months, the formula will only average the last 6 months. I know that typically someone would just update the range, but this is a particularly inexperienced group and I am trying to prevent phone calls every month to "fix" something when they just don't know how/want to learn.

Example:

See attachment for example...Thanks!Attachment 227138