Stain a half stained deck
You'll have to eyeball it. You'd stain the unstained &amp; wipe off the excess where it looks pretty close...test wiping here &amp; there. Then, let it dry &amp; see how close or far it is from the inside portion.

If it's too far or too dark, then you can sand it out a little, but that's a crap shoot &amp; it's better to just let it weather &amp; sun down by itself. If it's too light you just add &amp; immediately wipe off without waiting or waiting much at all. Adding stain to the inside portion will be very difficult to eyeball if it's not in the sun, so I wouldn't try doing anything there.