Hi there,
When you do something like TotalCount(ValueA) I'm assuming you're aggregating all of the ValueA values for the documents within a certain time range. To transform that value we'd need to run some kind of script on it. Unfortunately, we don't currently support scripts for aggregations. See Scripted field on Aggregated value for a similar question.
I'm a little confused by your question though, because it seems like the Percentage result you're looking for would be a single value for a given time range, and not a value which changes over time. How were you planning on plotting this on a chart? What would you like your X-Axis to be?
Thanks,
CJ