Hmm, tinkering with paint can make it impossible to match at a later date. You could get a quart of same color in flat and mix, mix, mix till you get the sheen toned down.

Years ago I was having a house painted and the sheen was to much. The painter used talcom powder to cut the sheen and it looked fine. He mixed it alot, and I can't remember how much he used. Perhaps you can google talcom powder and paint sheen. That paint lasted fine for 10 years...

Hope that helps.