Stupid question 
Not actually put petrol in, but get a full tank.
I usually just put in £10 or £20 worth but am wanting to get a full tank for a weekend away. If you have a rough idea of litres to fill do you just do that? The level isn’t always accurate on the dash board display though is it?
Or do you work it out in cost, say it’s £20 for around a quarter tank, put in £80 worth?