Rural North,Mid or South Wales? 
  
My V-Power round trip in Rural N Wales is 35 mile. (sometimes 45 because of extras  ) 
You really do just get used to the petrol filling thing. 
As G Man says when her light comes on, you probably have 18 to 20 Ltrs left so about 100+ miles. 
And as Ebized says you can have 'multi maps' so one for 95 etc. 
  
Have a word with Mark at Abbey MS or Jez at Horsham Dev. (both respected traders on here)