I am not aware of any way within the Histogram Platform to make the changes to the Percents as you have specified. Also, the percent objects on the histogram are not surfaced to the Frame Box() of the histogram. So, using the Customize to modify them is not available. However, a script could be written to calculate the percentages, and then apply them on top of the Histogram Frame Box(). It would be a bit of a challenge, but it could be done.
Jim