Gas gauge off after 996 cluster swap

[ChrisZang]

I swapped my 986 for a 996 cluster some time ago and generally everything works (all indicator lights and gauges).

Except there's one small issue:
My cluster thinks the gas tank has a capacity of 20 gallons vs. the 16 gallons it really has. Yes, that means it came from a C4S convertible

I understand that I need a PIWIS to reprogram it.
What are the exact steps in PIWIS to change the capacity of the gas tank but w/o loosing any other features (like cruise control, computer, etc.)
Do I need to do any calibration afterwards?

Thanks