If you are intending to make more than one cake and they are different sizes, consider putting them in the oven together. This saves time and also energy. Try the cake tins in the oven for size before turning the heat on. The tins should not be touching each other or the sides of the oven to allow the hot air to circulate round them.
The picture below shows three cakes I baked on the same shelf. They consist of an 8″, 6″ and 4″ cake.
Firstly I mixed the large 8″ cake and put it in the oven to cook. I then mixed the 6″ and 4″ ones using the quantities for an 8″ round cake. I then put these two in the oven. As they were approx 20 minutes later going into the oven, they were all ready to have the temperature reduced at the same time for the remainder of their cooking time.