Once you perform your unit turn with both hands on the racket,
the non-dominant hand comes off the throat of the racket and
extends out to the side (more or less parallel to the net) as seen in the image above. The off-arm will stay in this extended position for a little while (half a second to more than a full second) before it moves forward. As long as your feet are in a
semi-open or a
neutral stance at this time, you can
use your extended hand/arm to measure your spacing to the incoming ball. Your extended hand should line up, more or less, to the incoming ball. If the ball appears to be coming in toward your elbow rather than your hand, then you need to increase your spacing by adjusting your feet (one or both feet).
Take a look at the Federer video in post #5. After 0:46 to nearly 0:48, we see that Roger is extending his left arm to the side before his racket starts to drop (right after his unit turn). We see Kevin Garlington doing this about 1:18 and 1:30 in post #3. Andy M does this in the 2nd image below (after his unit turn). However his stance is nearly fully open, with
his upper torso is fully coiled, so the ball might not be coming exactly toward his outstretched hand. Try measuring first with neutral and semi-open stances before doing so with more fully open stances like this.