how to use video-crop with "add" keybind?

[geextahslex]

Hi, I tested different settings and the only keybind that is working with video-crop is set for example set video-crop 63%x85%
I would like to use it with add to be more flexible. Is this possible?
something like this doesn't work add video-crop -400:400:410:500

You get this numerator denominator error

Thank you :)