The problem with the "fuzzy numbers" is due to interpolation that the RRDTool code does.  From the RRDTool website,

Data Acquisition

When monitoring the state of a system, it is convenient to have the data available at a constant time interval. Unfortunately, you may not always be able to fetch data at exactly the time you want to. Therefore RRDtool lets you update the logfile at any time you want. It will automatically interpolate the value of the data-source (DS) at the latest official time-slot (interval) and write this interpolated value to the log. The original value you have supplied is stored as well and is also taken into account when interpolating the next log entry.

At the moment I don't think there's a way around that.

--Jay

RRA:AVERAGE:0.5:1:600

Looking at the above definitions, this is the one that I think needs to be converted to last.

I've gone ahead and make a wiki page with much of the information from this thread consolidated. RRD Experts please verify it is correct. This should help other people trying to do this common task, and once it seems mostly correct, I'll likely link to the page from the Community FAQ.

docs/DOC-4426