Take a glass you would drink whiskey from.
Extend the first two fingers of your hand, together. Place your fingers against the glass, with your middle finger against the bottom of the glass.
Note where the edge of your first finger ends on the glass. Draw an imaginary line there.
Fill the glass to the imaginary line.
Measure the amount of liquid. That's your answer.
When people ask for "two fingers" they are not asking for a precise amount. More than a shot. Less than a full glass.