NeophyteTheoretically, this should be doable with the percentile function. It's actually the only function that only works in graph virtual datapoints and doesn't work in complex datapoint expressions. That's because percentile needs a set of data to calculate against and a single poll only has the single poll.
The 100% percentile, should be the largest value in the set. The set should be determined by the timeframe of the report. The real problem is that the table only shows the current/most recent value of the datapoint, similar to big number. So, the "set" of data that the percentile function would use would be a single datapoint, which isn't what you want.
However, if you are already using line graphs for this, you could create a virtual datapoint with the expression "percent(datapointname, 100)". When you plot that, you should see that it will be a flat line at the highest point in the graph.
