I'm not sure you can do that directly in the DRO. There is an "on update" script that you can add code to. So you *may* be able to get the current (i.e. new) value, multiply by your scale factor and then somehow write that back to the DRO. I don't know if that would then trigger ANOTHER update event, creating an infinite loop.
I think what you would have to do is write a Lua function that reads the SmartBOB encoder register, multiplies that by your scale factor to convert to inches, and then update (somehow) a field on the screen. The script would go in the screen set's PLC script so that it would run periodically.
There may be a better way. I would suggest posting this on the MachSupport forums (
https://www.machsupport.com/forum/). There are a few people there who have done lots of things with Lua and may be able to give you some better suggestions.
Bob