I would definitely start off with a few small batches.
FA themselves recommend starting their watermelon at 2% - 3%.
I
vape 50pg/50vg FA wm 3% ~ 5% max, IMO it turns horrid past 6%.
Most FA fruits I've tried has a strong overpowering sour flavor when mixed at high %, of course this dissipates over time slightly.
I'd try it right after thoroughly combining ingredients & allowing time for bubbles to dissipate. Then try it every day, I don't steep any fruit flavors for any set amount of time, usually overnight to maybe 3 days.
Once I mixed a double by accident at 7% it took 2 weeks before it was vapable.
My favorite stand alone fa watermelon is 3.5%.
Works well with sweeteners if you like sweet.
works great with green apple 3.25% watermelon 1% green apple.
3.3% watermelon 1.25% strawberry.
3.3% wm .5 - 1% cucumber.
3.75% wm .07% pineapple.