There's various ways of ensuring correct chain alignment, and using the marks on the arm is not one of them!
I use a vernier gauge, others use string along the wheels, or rods which clamp to the rear sprocket and point down the chain line.
_________________
MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball