All of the flavors are going to be different in their strength. My best suggestion would be to mix up small batches to start. I've found the fruit flavors to be generally strong. Measuring with a dropper bottle is best, just be sure that you use the same size dropper bottle for each of your components so that there is some uniformity. You can then have a good idea of what percentages you are working with.
As a sample, I would take 5% of one of your flavorings, or 1 drop, then 95% of the dilution base, or 19 drops, shake that up in a bottle and test it. If it doesn't seem strong enough, mix up a 2nd batch, this time 10% flavoring, or 2 drops, then 90% of the dilution base, or 18 drops.
Lets say you decide that 15% flavoring is just right, you can then take a larger bottle, 10ml, add in 15% flavoring, or 1.5ml, then 85% dilution base, or 8.5ml. Those measurements can be done with a syringe, pipette, or my favorite, the graduated cylinder.
Also I highly suggest the use of the
ejuice Me Up calculator. It is a stand alone program that can be downloaded online and installed on Windows based computers.