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You can use whatever toolhead spacing you want. I've seen people with >100 mm toolhead spacing with Sailfish.

The +/-4mm relates to using a relative spacing vs. an absolute spacing. Owing to how MakerBot originally implemented dual extrusion back in 2011, the firmwares interpret any X toolhead offset in the range -4 to +4 mm as a deviation from the "ideal" toolhead spacing. The ideal toolhead spacing being a value compiled in to the firmware. But, if you enter in a value larger than +4 mm or less than -4 mm, then that value is taken to be the actual (absolute) X toolhead offset. So, you should just be able to enter 38.5 mm in which case you will then have that as the actual X toolhead offset.

Mind you, the built in calibration script will not work for you. But recompiling the firmware does not fix that anyway: you need to change the gcode for the script's source, convert it to X3G, then convert that the hexadecimal values represented in US-ASCII, and then add that to the firmware sources. Not really worth the effort. You can simply modify the gcode script, convert it to X3G, and then keep it on an SD card.