This works as I want it to, but I just don't understand how the settings used come about this.

http://page-test.co.uk/v.html

I understand the first symbol (class 'email-symbol') which is the same size as the image on the sprite sheet, but I don't understand the others.

It's 'background-position' and 'background-size' where I can't understand how the figures are calculated. There doesn't seem to be a pattern.

I want to get a formula so I can easily do this for all other images on that sprite sheet and also for other sprite sheets.

I don't get how background-size:1000% works for the one double the size and then background-size:1200% works for the one ten times the size.