Subtract value for word/character count

[Pavel Sanda]

Hi,

evil and external sources sometimes declare that I need to cut my document
by certain amount of characters/words :)
Mentally tracking the difference (e.g. 4587 - 150 words) while editing is
somewhat annoying.

I plan to add possibility for subtractive value applied to our statistics
function so I can easily see the relative counts and track my progress.

However I am not sure how much is my usecase obscure in general and
thus whether I should upstream some reasonable UI/lfun (preffered) --
or should I just go for simple hack for my personal patchset.

Objections if I add this? 
(For 2.5.x and 2.4.1/2 if Riki agrees).

Pavel