How much water for a 1000K attempt?

I am working on the planning for a 1000K attempt. One question I have
is how much water should I launch with in my Ventus B? I can carry up
to 40 gallons (150 liters), but for a 1000K flight I will need to
launch in early weak conditions, followed by strong conditions latter
in the day.

Options would be to launch with 20 to 40 gallons. Twenty to thirty
would probably give a good balance for early climb and later running.

Any suggestions for an optimum balance?

Thanks,

Tim