I would say that impressions are 'counted' rather than 'calculated' which to me, at least, implies that some formula is at work. Impressions are counted when the ad is seen. So, if your ad would have appeared on the third page of results but the user only looked at the first and second page - then no impression is counted.
Sometimes raising ones Max CPC can result in more impressions if it makes the difference between showing on the second page and showing on first page. This is because more folks look at the first page, than both the first and second.
Ordinarily, though, it is the Daily Budget that most influences the number of impressions.
AWA
