This app was mentioned in 5 comments, with an average of 1.60 upvotes
You have two good options:
SWApp True Dark: dim by an amount you choose by tapping at the 3 o'clock position (edit: probably should have read the whole post)
Wear Brightness Control: Slide along edge of watch to change brightness
Wear Brightness Control is perfect for changing brightness on fly by sliding on edge of display: https://play.google.com/store/apps/details?id=com.edgecontrols.android&hl=en
Best way I found to control watch brightness so far: https://play.google.com/store/apps/details?id=com.edgecontrols.android