Is it always necessary to ensure that you only treat the exact amount of petrol for that fill-up?

The reason I ask is that I very rarely put 50 litres of fuel in my car when filling up, the reason for this is the location of the only garage around that sells SUL.

Today is a good example, when leaving work I would like to fill-up (the Garage is next to the train station) but I think I will only get about 43/45 litres in the tank, if I don't I will have to drive 15 miles to get to the garage at the weekend (as much fun this can be I would rather not do it).

I use Millers Octane + which treats 50 Litres, if I continually only traet 45(ish) litres everytime I fill up will this have any adverse effects??

There are the odd times when I can get 50 litres in (if I happen to be running low mid week).

Cheers,

Dave