Feature Request:[+%][-%]settings

I usually set my [+%] value as 0.1%, which is convenient for me on adjusting focus and shutter.

But I found it impossible to define [-%] as 0.1%, and it shows that either 0 or 1 is effective.   

Is that a bug ? or I'd like to know why there's difference between [+%]and[ -%].

Thanks!

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