Sounds like if you use half a tank (50 litres) getting there, you have half a tank left for drifts and then 70 for the way home, just run efficiently as you can and don't blast back up the drift as the tide can be pushing hard. Just stick to you usual cruising revs. As already said, there is Portland marina close by for a top up. Using the plotter to reset at the start of a trip, and then calculating the fuel used is something you should know. Then if you do 1 / 1.25 / 1.5 / 2 litres per mile it's a quick sum and you know what is expected. Just remember, if weather changes you can use 30/50% more fuel if it gets really rough. Sunday I think we did 70/80 miles in total. R