I'm going to share with you a simple trick I use so that I don't have to deal with another heart palpitation again! Who am I kidding, heart palpitations go hand in hand with caking, regardless of how organised you are, haha.
However, this simple trick will help you remove the guess work and cut your time in the kitchen in half!
This is how I work out how much buttercream I'd need for a cake. Do this every time you make a different size cake and over time, you will have the data you need for all the sizes. You only need to do this once for each size, and then stop guessing for the rest of your life!