As per the discussion on Why do “Invalid Flag” flags get disputed?, the "helpful"/"disputed" resolution of "invalid flag" flags is giving limited or zero value on whether the "invalid flag" flag was justified. I therefore suggest to exclude "invalid flag" flags completely from the flag statistics, i.e. make it a seperate category of flags in the right column in the Flagging history page.
I'm not really a statistics hunter, but I keep an eye on my "helpful" flagging ratio to know whether I do a good job or not. "Invalid flag" flags corrupt this ratio and makes it quite complicated.