I found it better to start on single flavours and I would make up 4 10ml batches @ 5%,10% ,15% & 20%. Then vape them to see which percentage was best or adjust to suit.
When you have worked out that single flavour percentages taste like you can start mixing them together how you want because you know that as an example 5% Strawberry is a mild background flavour wit 10% Watermelon as the strong flavour
That's what I do when I can be bothered and make notes of the test batches like 5% no taste 15% perfect