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Most accurate poor man's technique might be taut string from axle to axle, drop plumb line from the top of the tube, measure length from top of tube to axle-to-axle line (hypotenuse), measure horizontal length to plumb line along axle-to-axle line or from top of tube to axle-to-axle line (opposite/adjacent) and work it out with trigonometry. Just need some string and a tape measure.
arcsin(B/A) probably gives you the most accurate result
Head tube is trickier as you have to project a straight line down to the axle-to-axle line (i.e. discount fork offset)
This would work right?
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frankenbike
Interested to know how you measure this. Head tube seems steeper than the seat tube to me (in the picture at least, could be lens distortion?) and Photoshop seems to agree (72 seat angle, 74 head angle).