Do you have the blend on hand, so that you can compare as you go?
I would start by using 5 drops of each of the EOs you know is in it, then start adding/counting drops of whichever seems to be the dominant aroma/s.
Sniff and tweak, sniff and tweak, is my method. It takes awhile, but it's fun.
Just write down whatever you do - I start with a tally mark system, then when I've decided I have what I want, count and write up a recipe for myself.