Degree marks indicate 2.5 degree of crank
It snaps onto the distributor, so when the clamp is loose it can't slip
Remove old plate and bolt.
Snap new plate around distributor and clamp as standard.
Find 0 degrease at 1000 rpm and mark the distributor shank with a punch.
Now readjust to your typical spec at 1000 rpm and count the marks. Now you
will be able to see the differenced while testing or reset back to
standard without a timing light.