The ferry comes in from Manhattan on the top right (the dotted line).
We have timed the walk down and it is only 10 minutes so we can easily catch any ferry if we leave 15 minutes before the scheduled departure.
The ferry, which is free, takes a half hour so we can be in Manhattan in 45 minutes.
The walk back home can take a bit longer since it is all uphill.
(Lots of hills in Staten Island!)