I'd prefer to tell the refueler how many lbs I order rather than a percentage of my capacity. I can imagine, it's easier with this preselection, but if possible, a window with exact amount of fuel to refuel would be great.
We can surely add it as a display option, so you would know how much a certain percentage is. However, it you want to enter it, just select the last option, which allows you to insert the quantity, using the default fuel page.