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I don't know how quickly air pressure changes
Rapid changes in pressure are accompanied by wind, so you'd never do any aerolabbing at times when the pressure was changing by more than 0.5mb/h. Taking a random* summer's day from here I got a total range of 1.03mb between 0800 and 1100 on July 15th last year, and 0.69mb between 1800 and 2100 the previous evening.
*Randomness not guaranteed - I picked the nicest looking period in the first half of July.
gbj_tester
There aren't really any calculations. You just google some air pressure value, estimate Crr value and then move sliders up and down.
But sure, the roll down tests, if you can get them repeatable will give you some idea about whether a change is working. Still need to keep an eye on the weather though - air pressure or wind changes during it might throw your results - I don't know how quickly air pressure changes. Xav might be able to give you more hints on doing roll down tests. The slevless forum and TTF probably have some decent discussions too.