I can’t say I’ve ever vaped it. I can say if you are so familiar w/ this juice you love you could perhaps pinpoint which watermelon is in it. You’re on the right track try getting smaller tester sizes of the few watermelons you feel are in it.
Watermelon FW- I really do not like this one but that is the thing w/ DIY taste is so subjective it’s unreal.
Watermelon - TFA/TPA ( same guys ) the flavor apprentice or parfumes apprentice. When we talk about it we typically keep the T in front of it since FA is a whole nother animal…FA = Flavourart and Italian Flavor Vendor.
Watermelon LA ( this is LorAnn ) you’ve probably seen juice that’s pink or that’s bright yellow well this original line does / did have coloring in the flavoring there is now at this time a colorless line for DIY ( yay! )
More on this one…this is a Candy juicy watermelon very commonly used as it isn’t overly expensive so you may try to get a tester size on this one too to rule it out and if it isn’t well you’ll have a nice candy like watermelon to work w/ later on.
Honeydew Melon TFA- This one has a unmistakable scent so you may just be able to rule it in just by your nose !
Honeydew Melon Caps( Capella’s ) this one isn’t as potent but I actually love them both
Hard Candy FW- I presume you mean this one as FA doesn’t make a hard candy to my knowledge.
Koolada TFA- is cool w/o mint taste
Spearmint & Peppermint TFA are what they are I don’t use either I prefer FA’s version of them…but since you are using Menthol I doubt you’ll have to be too choosy w/ the mints as they’ll compliment the menthol.
Candy Floss—Circus Cotton Candy TFA- thread lightly it really can mute the entire batch over time and while I am sure it contributes to the overall flavoring you may decide to add it after steeping has completed.
Fruity juices don’t take a lot of time , most of the time I say this because some do sometimes 
So once you’ve ordered your supplies while you’re waiting on them look up each of your flavors you’ve bought learn their % ratios that you’ll use them by.
http://e-liquid-recipes.com/flavor/18859 this is an example of what hard candy looks like on the ELR calculator page scroll down and look to the left there will be a suggested ratio this is where most ppl use the flavoring w/ good results so you use this as a guide and really try to stick w/ it once you get to know the flavoring you can push it outside of the ratio some , some if you use too much right away it is chemically and just very gross and will easily ruin your batch…trust me i’ve done this as I like to experiment all the time w/ how far can a flavor go.
While I commend you on your trusty 1 flavor profile when you get into DIY I am here to tell you , your mind starts going wild w/ the possibilities of what you can make …it’s totally fun and addicting !
Welcome and good luck !
corney but it is what it is !