Get a flat rimmed vase and fill it with warm water.
Place it in the tub and lay on it inserting your cock with your balls off to the side. The water will run out, and once you remove yourself, you can measure what is left over. Subtract this from how much it takes to fill the vase to find out the total volume of your shaft
Have your friend do it as well.

Then just for shits and giggles, you do it and then him seeing if any extra runs out and vise versa.

Then dry yourselves off and get dressed and go out and have a drink and laugh at how stupid this all was