Jens – if you’ve not got the headset in yet, here’s how to do it.
Measure the internal diameters of the headtube with vernier callipers in a few directions. If the measurement comes up round and all about 43.95mm press the cup straight in (this was the case on my BPX’s lower headtube hole). If the measurement is a little off (as per my headtube’s top hole) then take the bearing out of the headset cup. Press the cup only into the frame using a headset press and some grease on the cup and inside the frame. If at this stage the bearing fits in, great, but mine didn’t, so I took the cup out with a large bar and a hammer (a rocket tool is the correct tool, but hey) and the put the bearing into the cup. Then press the cup with the bearing in into the now loosened headtube (be careful to press on the cup and not the bearing). Use grease. Job done. Worked a charm for me and someone else I know.
Sure, its less than ideal, but really you should be reaming the headtube of any new frame before fitting a headset anyway and since you (or your mechanic) isn’t doing that you kinda have to accept life may be a little more fun.
Hopefully you’ll get things sorted so you can start enjoying the bike and forgetting about all this stuff :)