Average values from beginning/end of set time interval

I need to take the "average" of a PI tag starting @ 00:00:01 and ending @ 00:05:00.  But it isn't a true average of all the values in that interval.  I only want to use the first value of the interval (@ 00:01) and the last value of the interval @(05:00).  

And this needs to continue in that same fashion (e.i. 00:05:01 to 00:10:00).  Also, the trend/value needs to be "stepped".

I have attempted using a totalizer but with little success because I need more than one sample every 5 minutes: one at 5:00 for the first average and one at 5:01 for the next average.

Any ideas/suggestions?