Bit of a hassle but to minimise the chances of putting in an extra litre you could do the following.

1) Assume the car is 1/4 full

2) Add 5 litres of petrol. Drive 55 miles

3) When you arrive look at gauge - is it at/above/below 1/4.

4) Drive back and add 4,5 or 6 litres depending on point 3.

I would guess that 55 miles in a Micra would need about 5 litres, assuming 'normal' traffic. The full tank is probably 45-55 litres. But a reading of a quarter-tank is rather unpredictable, it could be rather more, as gauges usually err on the side of safety.

If it's the latest Micra, the tank capacity is only 41L (my K11 mid 90s car had a 42L tank capacity), but unless the OP intends to drive in very heavy traffic throughout their loan period, then I suspect they should get around high 40s mpg minimum, and likely around 50 or so in mixed driving.

Essentially 10-11L should be good for about 105 miles, possibly 120 with luck. I think though that they should err on the side of caution, even if the guages do so as well. TBH most garages only do a coursary check to see if the needle is around 1/4 - a bit above or below probably doesn't make that much of a difference.