The capacitor does not really affect the voltage provided by the 7805. Generally those capacitors are there to smooth transient response (response to square waves), but some regulators need them to be stable. (but not the 7805) You can look up one of the many 7805 datasheets for recommended capacitors. If you need a precise voltage you can use an adjustable lm317 and set the voltage, or you can just try a few 7805s until you find the right one.