Take US-101 North from SF to Central San Rafael, make a right on 2nd Street and follow that all of the way out to China Camp.

or

Take US-101 South from Novato and points north to San Pedro Rd/Civic Center in San Rafael. Follow San Pedro Road all of the way to China Camp.

The total accumulation of rain during the run will be less than 1/20" - basically nothing. You needn't fear getting drenched.