Almost all my concentrates are Fantasy Fudge extra strength food flavourings, and I measured the drops into a syringe to determine how many per ml.
As for mixing - whatever works for you. You're used to blending flavours, so it's quite an easy process for you (I find that my experience of mixing scents helps) - plus I think it's useful if you're quite a creative type anyway.
For ratios, I've tested & reviewed all the flavours on their own, so I already know which ones are particularly stronger or weaker than average. The first mix is done mostly on instinct and experience of each flavour - so I know that if nutmeg, cinnamon, Aniseed or cream are going into the mix, they need to be added much more sparingly than other ingredients to get a good balance.
I tend to start with a 5ml mix at about 10% flavour, so this allows me to increase and adjust it - so if one flavour is really overwhelming, I can increase the blend to 10ml and double other flavours to balance it out.
Once I get that first blend in the bottle, I judge it by sniff-test - if it smells OK, I'll add the nic & base VG/PG & test it straight away - if it seems close to what I'm looking for, I'll leave it at that and allow it to steep over a week or two (though if it already tastes good, I'll use a bit of it along the way). If first test is really strong or weak on one or more ingredient, I'll tweak it a bit there.
Based on how that first bottle vaped, I'll either make up more copying the recipe, tweak it a little on the next batch if flavour balance was a little out, or abandon the project if the idea turned out to be not my cup of tea.
I find this works for me, and results in more successes than failures
When looking for inspiration in the first place, I have sometimes tootled through a few cocktail websites