Why not autofocus and then lock?
I guess what I am asking then is, how do you do that with chdikptp? 
set_aflock(1)
should do an AF scan and lock focus.
Alternatively, you could get the camera autofocus distance using
=press'shoot_half' repeat sleep(10) until get_shooting() return get_focus()
And then pass the value to your shoot commands with the -sd=<number>mm option.
Or you can use
=set_mf(1)
and adjust the focus with set_focus until looks good.
If you trigger your shots with rsint command, half press is held between shoots, so all settings will be kept the same.
For your rig, you may be able to find the best focus distance once (perhaps starting with AF as described above, and then tuning with jpeg sizes if needed) and then just use that value every time you use it, at least when you aren't doing books that several inches thick. Cameras like the elph180 have a pretty large depth of field, and relatively coarse focus adjustment. [/code]
