I just asked me in which size i have to shot the angle to resize it perfectly in 400x290. Any suggestions?

What's the max you want (or can) shoot in?
Then divide that resolution with the current scale, per side.
E.g. if I wanna shoot in 1600x1050:
To: 400x290

(1600/400)*290 = 1160 --- my height isn't that high :3.
(1050/290)*400 = 1448,2758620689655172413793103448 --- yay
So I shoot my screenshots at exactly 1448,2758620689655172413793103448x1050.

thank you

edit: silly me