Here's an example of drawing out the purlin lip cut angle, using the Draw Down Technique. As long as the rafter is perpendicular to the roof surface you can use this Draw Down Technique as a base drawing for anytype of rafter parallel to the eave line like purlins or any rafter skewed from the eave line.
In this drawing the purlin rafter in plan view is transferred to the Draw Down Roof Surface using perpendicular lines.
With the timber laying on top of the purlin on the roof surface, draw the perpendicular lines on the sides of the timber for the miter angles on the sides of the purlin rafter and connect the lines to draw out the back bevel lines on the top edge of the purlin.
Transparent view of the timber showing the miter angle planes of the purlin.
Transparent view of the timber showing the purlin lip planes of the purlin.
The claw lines in plan view need to be continued up to the roof surface. It can be confusing as to which claw line from plan view to use to develop the purlin lip / claw on the timber. Draw back bevel lines on top of the timer to make sure your purlin lip/claw lines develop a back bevel angle the same as the miter angle back bevel and the lines are parallel.