Edit function is wacky. Sorry for the added reply.
The other way to do this if you don't want to bend your brain with trig is to simply draw up a small scale diagram. Like 1/8 inch per inch of the full size model. Draw a line out that is the scaled length of the half span. To that you would extend the line by the wing chord or spar to TE length at the washout angle. In this case they are the same so just make one line that is the span plus the chord or partial chord. With a line parallel to the base or "0°" line that you have coming off the center end of the "wing" line extend the end of the span plus chord end back over the tip end of the span line. That's your added height due to the washout. Draw a new line back to the center line end of the wing and measure the new angle.
Likely as not the difference will be less than 1/2 a degree.