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Tijs-
With a flat toptube then you can easily work out the head and seat angles. If you have an iphone, use the angle finder, zero it on the top tube and then you'll get the head and seat angles straight out the phone (watch out for side buttons. My old SE had one perfectly flat side but new 2nd gen SE has buttons on either side).
If you have any cad software or are good with scale drawings you can take those angles, measure all the tubes and draw the basic double diamond shape up, doesn't need any detail, just lines, then you can extend the headtube line down, add in your rake and measure the a2c.
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I meant to do it in real life really but as M_V says, you could draw it out in software pretty easily with a few measurements.
M_V
PhilDAS
If you know the BB drop, you could draw or plum line from the rear dropouts to the front axle fairly accurately. If you know the rake too then you should be able to draw a line through the headtube to the line that intersects the axles, add the rake and measure the axle to crown that way after accounting for headset stack.