If you do a "40 drop" flavor test and add more than 1 drop of flavor, doesn't this change the percentage of your base mix with each drop of flavor you add?
Yes, your base will get progressively weaker. This is just testing, to give you the rough idea of what the flavours are going to do. You just have to keep in mind roughly where you are at. Once you've got a better idea, start another drop test, so say you are doing a 20 drop test and want to try 3 three drops (15% in this case) and 1 drop (5%) then you go 16 drops of base, 3 drops flavour A and 1 drop flavour B and so on.
I'm a little confused as I've only been DIY'ing for a week or so.
Don't over think it. This is shoot from the hip easy DIY. Do take notes though, it's easy to get mixed up as to exactly what you did. Before going to a larger batch it might pay to do another drop test starting from scratch, just to verify what you think is correct. I recommend (if I haven't already in this thread) that you shoot for a weaker flavour overall. Really strong flavours can be nice, but after half an hour they can get sickly and turn you off completely.
You say 1 drop is 2.5%.... So does that mean if you add 10 drops of flavor, you're at 25% flavor and 75% base mix?
That is correct for a 40 drop test yes, 10 drops of flavour, 30 drops of base. One drop in a 20 drop test is 5%. 1/20*100=5% or 1/40*100=2.5% for a 40 drop test. Some people just prefer to work in ratios first - 1:20 3:40 and so on. It's easy enough to convert to percentages later.
Once you think you have something make up 10ml of it and try it out. I recommend you try it in these small batches for up to a week before you go larger. I have mixes that I decided that I didn't like after 3 or 4 ml, and others I wasn't sure about until I'd made up 3 small batches.
Also, at this point you are going to have to know exactly how many drops you get to the ml with your equipment. Juice calculators usually assume 20 drops/ml but this is hardly ever the case. Just look at your 20 drop test or whatever. A whole ml? possibly half a ml or even less. I work on 50 drops/ml as tested by dripping PG into a 2ml syringe and then halving the number of drops to get the number for 1ml. It depends on the size of the dripper, the temperature and viscosity.