Baking IS a science.....
- as Emeril Lagasse said, "it's a formula, don't mess with it".
Back in the Dark Ages, some of us did learn ratios about the amount of "raising agent" to flour and shortening to sugar. The beauty of American Standard Measure (cups/spoons) is that it's easier than weighing, and less subject to goofs if you're just pouring flour into a bowl that has other stuff already in.
I hate fan ovens for baking. About 21 years ago, I bought a little book specific to fan ovens, but still find it doesn't quite do the job. Using the mantra "lower the temperature by 20 degrees" is fine as a general guide, but baking is best in a static oven. A true convection oven is sort of o.k. IMHO, but few of the 'fan ovens' here are true convection ovens.
I keep a temperature conversion chart handy, but even after all these years, I still have to watch to make sure whatever I'm baking isn't rising too fast, browning too much, or worst yet, NOT doing what it should....like a pumpkin pie that doesn't "set" properly.