With each pound of pressure, the boiling temp of water is increased 10 degrees. The Boxster's cooling system is pressurized to 19psi. So, use extreme caution when removing the coolant cap when engine is warm/hot. Cover the cap w/ a towel when loosening (you'll most likely need one anyhow since it will be H-O-T hot! Anyhow, from the Bentley manual....
- For tiptronic, remove fuse B1 in fuse panel to disable ATF cooler shut-off valve)
- remove filler neck trim cover
- open up bleeder valve locking clip
- fill w/ coolant to bottom edge of filler neck
- run engine at idle. Top off w/ coolant until no more can be added when engine idle is raised sharply
- replace coolant cap and tighten. Warm up engine for about 10 min at approx 2500 rpm until coolant thermostat opens. To be sure engine is fully warmed up, check to see that radiator cooling fans in front wheel wells are on.
- allow engine to run addl 5 minutes at 2500 rpm. Every 30 seconds briefly rev engine to 5000 rpm
- open reservoir cap, relieving coolant pressure w/ extreme care. Top off coolant, close cap and repeat process of intermittently revving to 5000K rpm for 5 minutes
- allow engine to idle until radiator cooling fans cycle on and off once. Shut off engine and open reservoir cap, relieving coolant pressure w/ care
- top off coolant until level reaches max
- flip down bleeder valve locking clip
-replace filler neck trim cover and cap.
- If tiptronic, replace fuse B1
Good luck
NOTE: the cooling system has a capacity of 18 liters IIRC and mix is 50/50 coolant w/ distilled water. As such, unless you've added a bunch of water recently, you should be ok to just use distilled water for the top offs (anytime as well as those called for during the bleeding process.)