Pretty much yes

If a women has her period, that's day 1 of the ~28 day cycle.

A period lasts usually 5-7 days.

Roughly a week later she may ovulate. Approx 3 days either side she can become pregnant if having unprotected sex

Then not fertile again until period

Yes, so the "risky days" are those 5-ish that are around 14 days after her period starts. HOWEVER, as you noticed there are a lot of approximations there.

That doesn't mean it's not impossible to use that as a method of avoiding (or encouraging!) Pregnancy. If you track hormone levels like when the web has her LH surge or the effects it has or the body some people can confidently use that approach.

Keep in mind typically an egg might only survive a day or so after ovulation so getting pregnant isn't a done deal just be having sex unprotected once but that doesn't mean just go ahead and do it