I would like this mod to allow me to limit, rather than extend my reach. Yes I am absolutely mad, mad I tell you!

It seems to me that allowing far reach unbalances the gameplay, allowing you to manually place items when you really ought to be using drones. So, just to make myself a challenge, I would like to reach only 1 tile in each direction. I think therefore you should be allowed to choose a reach bonus from -6 to 123.

Sadly, I have just answered this question for myself and the answer is "Not possible."

character_build_distance_bonus must be >= 0 where 0 gives you a reach of 6, so anything less than a reach of 6 is impossible.

I've made a mod to address this shortcoming. Short Reach https://mods.factorio.com/mod/short_reach can reduce character_build_distance_bonus to zero. And it's actually compatible with Far Reach too.