lowering ride height by spring perch also decrease damper travel ?

My question is does turning the spring perch downward to lower ride height will also shorten the damper travel ?

No, the damper will travel the same amount (it just starts at a different position, so you will have more droop travel and less bump travel, but the overall travel is the same).

Thanks for the answer , it make sense.

How do we make choice (compromise) by lowering ride height vs shorten bump travel ?

Also how do we know if more droop or more bump travel is more suitable ?

Usually damper length and travel is a packaging issue. If you lower the car enough, you many need to shorten the dampers to make it all fit.

Generally you shorten the dampers to reduce the amount of droop. This can sometimes be done internal to the damper by installing one more spacers to limit the extended length.