there are several way to go about doing this. one simple way might be to:how do I figure out if I need to alter the file to fit my large head and, if I do, how do I go about altering the file?
1. I think the beasurement 8.5 is the length of your head (front to back)
2. convert to mm - 215.9
3. use the measurement tool to measure the internal distance
4. if you dont know how to do step 3, please search youtube for "the hero tutorials" and review the series
5. do some math 215.9 x (current scale) / (internal measurement) = new scale.
6. print, build, adjust as necessary. it is normal for these things to have to be built 2 or more times. especially when you are starting out.