I tried a similar exercise a few years but adopted a different approach. My definition of "flyable" was simply "did anything fly on that day?". This was based on statistics from my local gliding club (Booker, so not that far from Denham). The thinking behind this is there's generally glider pilots lurking on the airfield who will try to make the most of the day and so if presented with an opportunity to fly they usually will. Admittedly gliding training parameters are slightly different to that in the power world, which perhaps provides greater flexibility - a 1,000' cloud base isn't a problem if you're doing circut practice and snow showers often have very exciting updrafts that you can soar along and stay aloft without the need for more traditional thermals.
I don't have the statistics to hand but I do recall a few important points:
- You will always fly more if you look at using the conditions presented to you on the day rather than having fixed definitions of what's considered "good enough"
- You will always fly more if you look at the weather after you've arrived at the airfield rather than looking at the forecast to decide whether to travel to the airfield in the first place
- marginal conditions, when accompanied by an instructor, provide a greater range of learning opportunities than "nice" days
- Like fishing, you should have been here yesterday. Or tomorrow.
- There's seven days in the week. Two are weekends, therefore if you can only fly at weekends you've instantly limited yourself to 2/7 of the flyable days
Cheers