That is true, but "better than 5 minutes ago" and "enough" could easily be very different things. Especially in your case where you are, at best, guessing based on your assumptions. Assumptions such as;
Which I'm assuming you've read (probably in the manual) yet completely ignored the preceding "up to". Which means it can be less than that but it can also me more than that, it's not the definitive figure that you car will be using that you seem to think it is. You could easily have used 1.8l in that 800miles you've owned the car, so even if the previous owner decided to top up the oil the day you bought it, you'd still have less in it now than you did then, so you actual don't
nor
I appreciate that you've had a stressful day with this but really listen to the advice above and stop driving it. If you can't get an accurate level reading, change the oil and start from scratch, if you can't do that yourself then enlist the help of someone who can even if it means trailering it to a garage.
If you do top it up yourself, take regular readings to ensure that you're adding the right amount, don't just guestimate. And leave the dipstick out whilst actually filling too, you'll find it easier to get a clean reading that way.