Garage_Woodworks wrote:

When calibrating tools 'eye-balling it' is NEVER good enough in my shop -
wooden or otherwise.

In lots of situations, the precision of decent eyeballs and finger tips
is actually very useful for checks.

Two examples:

Thickness planer parallelism - plane a board, cut it in half, put
opposite edges together, rub the fingers over the meeting point.

Table saw blade or miter gauge/sled 90 degree accuracy - Cut a board in
half, flip one board, place the cut edges together and check for gaps.

If they don't work out, the calibration tools make recalibration easier
and faster than more test cuts, but the cuts are good enough for
in-service spot checks.