It all depends on the flavors you use. There are flavors that get weaker over time and there are ones that get stronger. This is why you should SFT (single flavor test) all your concentrates to get a feel for it. Bakery and custards will give you more mouthfeel over time and the flavor will intensify. Fruity flavors are (generally speaking) more volatile and lose potency over time… but there is no one-rule-fits-all. If you’re talking about Lemon (MF) for example, it will need a month or more to reach its full potential.
If you have a recipe that you like (but it’s not strong enough), make a one-shot flavor for it and test that at different strengths. The recipe you gave is 12% total flavor, make one at 16%, one at 20% and one at 24% and see how that works out for you.
Using more flavor will initially increase the flavor strength but at a certain point (depending on flavor concentrate and brand) it will lose its potency. I’m no MTL user but I’ve always heard you should use a little more flavor.
Watered down recipes are, in my experience, either under flavored or in need of more steeping (or both), at least if you know that you get good flavor from your setup with other juices. If everything is watered down then something’s not right with your hardware or your taste buds/smell.