Ran into this issue today.
If you agree that 50 * 100% = 50 and that 50 - 50% = 25 and that 50 - 100% = 0 then there is a bug in the way Ctrl-J adjusts by %.
Its actually only adjusting by logrithmic factor of the value you put in there. (Non-linear).
For example in cam angles... under the High Map if I multiply a cam angle of 50 by -100%... I get 25.
If I subract 50 -200% I get 16... etc etc. FPMv:1.1.2.7
Steve