sweet spot
A particular point, area, or range where something will be most effective or beneficial. The tension is in my shoulders. Move your hands a little higher… there! That's the sweet spot. There isn't a very strong signal out here, so you have to move the dial until the radio finds a sweet spot.
the sweet spot
a particularly fortunate or beneficial circumstance or factor. The sweet spot on a tennis racket is the point believed by players to deliver the maximum power to the ball. In 1997 a physicist in Australia claimed to have disproved its existence.
1997 Times Enjoy the ‘sweet spot’ now, but don't expect a boom.