The math is pretty simple based on your parameters, you might be thinking about it too much.
Starting with the fact that every triangle equals 180° total angle measure, and you have 2, you can figure out the 3rd.
Target bearing 26° with an AoB of 131° on the port side gives you a resultant angle of 23° (180 - (131 + 26)). Since he's headed from right to left, that will cause you to subtract from your True Heading of 284° (284-23) which gives you a heading of 261°. So your charting was more or less correct (+/- 1°) based on the input parameters but it's possible that your input parameters were off.
It's also possible that the contact changed course.
At any rate, that's the basic math behind it, solve for the angle that your contact and ownship make where the paths cross, then subtract or add based on which side the contact is headed. Of course if you have access to the attack disk, you can solve it without doing any math or charting.
You can also plug in your parameters into the TDC. Ie. Rotate your periscope to the target's bearing, set the AoB, lock the TDC and rotate your periscope to relative bearing of zero (straight ahead) and that will give you a new AoB, in this case 157°. Since he's headed away from you, subtract (180-157) equals 23°. Either way, still doing math I guess.