As Denis implied, your accuracy will only be as accurate as your data, thus a monthly (or daily) average may not mean much. However, there are several ways you can get a daily average and part of that is the other aspect of what Denise said. Just what does your data mean? Let's assumed the numbers refer to the 'items on hand' [or, if you prefer, number of customers or ...]. Does Jan-76.1 mean that in January I had 76.1 items on hand each day or does it mean for the month January over the last 'several years' we held a total of 76.1 items. If the former, you already have your answer and it would correspond to Antonio's answer for a daily average for the year.
However, if it were the latter, you could divide by 31 to get the daily average for January. Then do the same thing for February, March, etc and then average them to get a daily average. Or you could just add them all up and divide by 365 or well there's probably a few other ways also.