This is very common framing in my area also.
But it is not a "standard" that can be determined by a formula in SoftPlan.
The purlin locations may vary per the framer and the bracing members must run from the purlin to a bearing wall (which is not defined for SoftPlan).
These are typically framed in my area with 2x6 material. The purlins and braces might be double "L" 2x6's.
An arbitrary formula could be created that used the ridge length times a factor (possibly 2) and called "purlins"
And another arbitrary formula could be created using rafter length times a small factor (1 in 10) called bracing.
I use the phrase "arbitrary" because they are not based on any "real" framing variables.
They are simply used to estimate extra materials for these framing functions.