I've just returned from my secret laboratory, where I checked the volume:
In liquid mesasure, the amount is precisely--ONE FINGER.
Like Planalp, I've misplaced my conversion tables; don't know what one vertical finger-width-of-sump-volume corresponds to in any other units of measure (a vertical finger-width of shot-glass volume is better known).
Regardless, I think expressing the quantity in mililiters, or cubic centimeters, will be more useful for purposes of precision than in ounces, avoirdupois.
Last edited by Damocles; 07-23-2013 at 08:46 AM.