Query with fill ups

[Dukey]

I keep track of the miles covered for each tank of fuel but am not sure how this is entered onto Fuelly. Lets say I filled up today and I know I covered 400 miles on the tank that just emptied. On the Fuelly site, would I put in the fuel details from my previous fill up receipt? Otherwise it will be telling it that I covered 400 miles with the details of what I just filled up with, rather than the correct amount/cost of it previously?

So as I filled up today, I will be putting in my previous slip and keeping this one until I've emptied the tank?

Thanks.