Your recipe calculator is one of the finest contributions to this forum IMHO. I do have some suggestions, but feel bad to even ask since it's so nice as it is. Do what you wish with them (implement them or not), I won't be offended!
1) For drops, the sheet assumes 20 drops = 1 ml. You could have a box where the user can enter how many drops his/her dropper takes to equal 1 ml and use that in your equations.
2) For flavorings, often someone will say: 60% this, 30% that, and 10% other. This is obviously getting much more complicated (for you), but for the flavorings, you could support up to 4 or 5 flavors and be able to enter what percent or "parts" for each one with an extra box to "name" it so the user doesn't forget which one is which.
3) Maybe a separate section for plain PG and plain VG? A while back I asked what ratio you used and you said 80/20 (PG/VG). I plan to just pre-mix that and use for each recipe. Of course, if I have 48mg PG flavorless juice and make recipes, the ACTUAL end ratio of PG/VG will vary depending on how much of the nic juice is needed which depends on what strength each batch is, if you follow me. So a person could say, "I like a ratio of 80% PG and 20% VG. My PG nic juice is 48mg, and I want 20% total flavors. How much flavors, plain PG, plain VG, and 48mg nic juice do I need to keep the 80/20 ratio and end up with 15ml of a 12mg batch?
See..... now why did you ask for suggestions? I honestly feel like these suggestions are almost out of line and I owe you an apology! I think the reason for that is because your calculator is so handy that I can't think of any simple thing you could change to make it any better. Well, except maybe #1. For #2, even I could probably figure my way through that on my own if needed. And of course #3 is just ridiculous. While the end PG/VG ratio would change slightly in different recipes (strengths), I hardly think it would be noticable unless you were making it by the gallon!
One thing that is nice about your calculator is that it is easy to use. No sense in making it over-complicated.
Great Ideas let me see what I can do in the next couple of days!
Dan