Section 4.8.2 on DARKTIME states that the time required to erase the CCDs before an exposure is 13.6 seconds. This appears to be incorrect. The WFPC2 IDT documentation (both Holtzman, et al., 1995 and the Instrument Description and User Handbook (JPL D-11212)) state this time is 16.4 seconds.
Hence all time values of 13.6 seconds in WFPC2 Handbook section 4.8.2 (pp 73-74) should be changed to 16.4 seconds.
This error should have no impact on 99.9% of observers. It would contribute a calibration error of 0.002 DN in the background level of a 1000 second exposure.