AI've been asked to do a wedding cake and the girl wants the same cake I made at my wedding but tiers in a different order I have no idea how to adjust my recipe from a 30cm cake to a 20 cm

I haven't tried it yet : hope this help

http://www.cakebaker.co.uk/baking-tin-size-conversion-calculator.html
 

Also found this:

http://culinaryarts.about.com/od/measurementsconversions/a/scaling.htm

Also could use the baker's %:

http://www.kingarthurflour.com/professional/bakers-percentage.html