The yesterday's score is just that... it's not additive. Joeri is right, you'll need at least 3/4 flights maybe depending on your previous trend to keep the score from going down. The score isn't additive, it's basically based on a rolling average which is logarithmically scaled according to the other factors.
I will look, it's on my list, but it won't be until next week at the soonest I think. I'm a bit slammed with moving the bug tracker over (the old one just got hit with a spam attack, had to clean it), and also with 2.1 being released this weekend.
Actually, if you're averaging now 4 flights a day and it levels down to about 2 flights a day, you will lose some points because that average is decreasing. But since it's rolling, it might be by a few points (max). If you go from something like 3 flights a day to 10 (40 day rolling average), then you will see an increase. But increases for only a few days here and there don't account for much unless they are sustained. That's where it account for the overall activity. So you can't do a week of 10-15 flights a day and then 5-6 flights a day. But in the end, you can do something like ((flights in the last 40 days / 40) * some variable weight) to give you the score
I'm planning to also have more stats per airline with the rolling averages and some detailed information about that so then points esp are a bit easier to see how they come. But I've just been getting slammed these last few weeks.