Editing a G-Code file is simple - open in a text editor (not word processor...) and add/change the G-Codes you want/need. I thought it was daunting the first time too, but then realized it's the same as turning the dials on a manual machine and keeping a copy of a G-Code reference (like the section of the manual for EMC from the linuxcnc.org website) handy to sort out what the program is already programmed to do and to decide what to add in to accomplish what you need to change. My more complex programs have had more hand coded information than CAM generated. Using free/shareware/opensource CAM I am the "post processor" adding in the environmental conditions I want/need (like turning the spindle on, coolant, pauses for the spindle to attain speed, pauses to let the spindle take a break on long programs, etc.). It's really a lot less complex than you'd think. If you do something really wrong, the problem will be identified (dialog box saying something like "Invalid argument on line xxx") when the file is opened and parsed.
If your screws are metric I would stay with a metric setup rather than a lot of math conversion and then using the metric/imperial workspace environment settings as suggested above.
Greg
Every day is a learning process, whether you remember yesterday or not is the hard part.
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