Geeetech

Hi all, new to the forum.
I've just "upgraded" my Geeetech I3 Pro B with a GT2560 and a 3dTouch probe. All is working well except for one issue that I am unable to solve within the firmware.
When probing the bed to level it, the probe goes off the back of the bed. To remedy this I changed the maximum probe parameters in the firmware, however when I try to change the probe offset numbers, specifically the Y probe offset from the extruded, I get an error message. This only occurs when I change the value to a positive number, it does not occur with numbers that are zero and negative. It also changes to the sanitycheck tab when the error occurs.
I have attached a photo of the error and the firmware saved file as I am not all that good with coding and changing of firmware. Any help is appreciated.
My printer has a print size of 200x200x180 and the probe is offset by 4mm to the right of the extruder when facing it and 40mm behind the extruder.
Thanking anybody who can help in advance, again much appreciated.

*UPDATE*
So turns out I'm really dumb, I wasn't following instructions close enough, so for anybody that has this issue in the future, read what is said and actually do the calculations instead of assuming the code was doing it for you, like I did. So this was my solution.

Thanks for your feedback.