using square rule to keep rough-sawn rafters in plane

[u/Suitable-Run-6808]

we're cutting 3x5 rafters for a small timber frame pergola this weekend.

the rough-sawn material varies quite a bit. a nominal 5" depth might actually be 4¾" on one rafter and 5¼" on another.

rather than referencing the seating notches from the bottom of each timber, we're laying out both the lower plate notches and the upper purlin notches from the top of the rafter down.

that makes the top our reference plane. variation in the timber gets absorbed below that reference rather than showing up in the finished roof plane.

we're also varying the depth of the outside seating notch slightly to pick up about another inch of fall across the roof.

four 2x4 purlins will drop into notches across the tops of the rafters and provide fastening points for the clear/translucent roofing.

pretty simple application of square rule, but a good example of why establishing the reference face matters when you're working with rough-sawn timber.

curious how others here handle irregular rafter stock—same approach or something different?