How do I add gas and oil to a gas can half-filled with 50:1 mixture so
that it stays 50:1?

For example: I have a 10 gallon can on my boat filled a little more
than half way with of 50:1 mix. I can't tell exactly how much 50:1
mix is in the can. I am at a gas station and want to top off the can
and yet keep the mix at 50:1. How would you do this?

Here are some possibilities I can see:

1. Put in a known quantity of gas that stops short of filling up the
tank. Calculate how much oil is needed for that known quantity of
gas. Add the oil, shake the can. Accept how much empty space there
is at the top of the can.

2. Fill up a another can with maybe 6 gallons of gas and add the
correct amount of oil (5/8 of a cup,.. or 10 tablespoons maybe).
Shake up the can and pour it into the 10-gallon can.

Any ideas?

Thanks

Mel

mricflo at dr dot com