How can I reduce sheen with touched-up wall paint?
My 2 cents...

I had the same issue and the painter told me the best course of action was to paint that one entire wall.

His reasoning was he could get the color very close, but never perfect and there was a good chance my eye would see the touch up.  Doing the entire wall fakes your eye into seeing the color as a whole.  In my house, my eye can't tell the difference so it was worth the money. 

Caveat:  Obviously you can't paint one side flat the other gloss, etc.