My FCD generally begins to be off when it displays over 60, otherwise it's pretty accurate up to that point.
For example on my 69.2MPG calculated tank it said 64.
In the 66 calculated tank it says about 62.

Because of that I know that if it says low 60's I am doing OK.

Right now it's at 61 point something with 7 bars of fuel left so I'm a little behind, thanks to a relentless driving wind. I'd guess my calculations will come out to about 63-64 in the end.

Efficient drivers do it better.
1003 miles a tank personal record. 74MPG calculated. HCH1 CVT