Partial fuel-up - when do you reset your trip odometer?
 

Quick question on a partial fuel up. What mileage should be entered on the partial and the subsequent full fill-up? If I drive 300 miles, then do a partial, and drive 100 more until a full fill-up would I use 300 & 100 respectively?

Yeah, Fuelly is expecting you to reset your tripometer with every fuel-up. If you have to do a partial fuel-up, just note all the details and mark the checkbox next to "This is a partial fuel-up", and reset your trip. Here's some more info about partials: How do I account for partial fuel-ups?

When I do a partial fill-up I always add a nice round volume of gas eg: 10 litres. Then on my next fill-up I just note "+10 litres" on the receipt.

In this case I wouldn't enter the partial fill-up because I will account for it on the next full fill-up manually. This will, however, mess-up the 'Best distance per fill-up" stat, but that one isn't as important to me...