Consider using a cookie for that purpose. I was looking for the calculator everywhere until I realized I had to enter my email first. That's a pretty big trust hurdle – no thanks.
I suggest you let people use the calculator freely, and when I enter e.g. "4 transatlantic flights per year" then you show me what that sin translates to specifically for one of your offset projects, e.g. "1000 briquettes" (with photos, of course). Personally speaking, that would make me pay up.
Oh, and the 20% fee is really problematic. I'd like to see a pay-what-I-want slider where I can adjust the percentage. The slider minimum would be set to min(0.2*x, $10) or some other reasonable amount, and you can calibrate that function to maximize donations.
I suggest you let people use the calculator freely, and when I enter e.g. "4 transatlantic flights per year" then you show me what that sin translates to specifically for one of your offset projects, e.g. "1000 briquettes" (with photos, of course). Personally speaking, that would make me pay up.
Oh, and the 20% fee is really problematic. I'd like to see a pay-what-I-want slider where I can adjust the percentage. The slider minimum would be set to min(0.2*x, $10) or some other reasonable amount, and you can calibrate that function to maximize donations.