For the price of two 60ml bottle of branded E-juice, I will be making or rather mixing 2.5 litres worth of liquid on my own. I'm not scared, since I already mixed PG+VG+NIC in the past. I'm just worried that I will just throw away the results if I mess up adding the flavorings and end up tasting like crap.
Since you are unfamiliar with these new concentrates; I would kindly suggest, that you start by mixing small single flavor testing samples (aka. SFTs = Single Flavor Testers). This will give you the opportunity to discover what percentages taste best to you... without wasting a lot of product.
@DaveP's suggestion, to start with single flavors, is sound. It allows for better understanding of what each concentrate is (or isn't) bringing in way of flavor. Once you get the individual "best"(for you) percentage is, you can even use the leftovers of the SFTs, to experiment with various flavor combinations, by combining them. This will even further conserve your supply.
As you may not have seen guys, I may be far far away from where you all are.. How much do you even pay for your ingredients anyways? I know cheap or expensive is very subjective, but I feel I got them dirt cheap... Or not?
Regarding what we pay here in the states; it varies a bit, depending on concentrate brand and supplier. But, as an example, a 4 ounce bottle of basic TFA, will run about $7.50. So, if you are getting something similar for $4, then yep... I'd say that's a pretty good deal.
When you receive them, let us know who the manufacturer is, and we might be able to offer some possible starting percentages.