Suggest you read the Pelican info on refilling the coolant. It can be tricky. Look for the Pelican diy project on replacing the w/p.Pedro's Garage also has info.
There are plenty of examples of Boxster owners thinking they had filled with coolant but were actually very low.It may be a good time to replace all the coolant anyway. 5 years is the max recommended by some. It is a serious issue to attend to with lots of info and care. Easy to screw up unless you have this equipment:
Airlift