Your Own Bills Edit
The formula for Clout Gain on your own bills appears to be:
BASE COST + CHA x 10% + INT x 5% + ATT x 10%
Where BASE COST is the COST of the Bill to the party of the same colour.
Note that the Clout Gain is determined at the moment of passing the bill, so your stats only matter at that exact moment.
Other Congressperson's Bills Edit
The formula for Clout Gain on other Congressperson's bills appears to be:
Number of Points Played On Bill x 6.6%