I am still very very new to DIY with only 4 different experimental batches under my belt, but so far the only thing I have "wasted" is I accidentally dropped a bottle of black cherry on my carpet. My rooms smells like cherries lol. But that being said, My first batch I just "flew by the seat of my pants" and dove right into it by adding and tasting it as I went. I get a feel for the flavor (I am wired) by mixing a few drops in pg (no nic) and taste it. I think that it gets a good place to start with how strong it is, and in effect how much to use in your juice. The marsh-mellow I have is almost flavorless, where as the raspberry I have is super strong. That being so, I know I would need a lot (more then 10%) of the marshmallow to get the taste I want, and way less of the raspberry (used 3%). These numbers I plug into the ejuice calc independently of each other (I have not figured out the program very well yet) and figure out how many drops I would need and go from there. That is what I did with my marshmallow berry mix (blackberry, raspberry, cherry and marsh-mellow) and it turned out ok. I have not got a good feel of it DIY yet, but I think the learning curve isnt as bad as I thought it was. Once I get all my flavors that I ordered in the mail, I am just going to go full force and learn.