Probably the most important thing to note is that 100mg/ml makes it pretty easy to calculate the dosage you want in your juices, because you can express your preferred dosage as a percent.
So, if you're making a 10ml bottle and you want a 12mg dosage using 100mg/ml nicotine liquid, all you need to do is add 12% (1.2ml) to 10ml of liquid (after you've added flavourings and any other additives) and you're done. For a 30ml bottle at 18mg, you're adding 18%, or 5.4ml in 30ml of liquid. Since you diluted some to 50mg/ml, double those percentages and volumes when using your diluted stuff.
Generally though you want to calculate how much you need in a given volume of liquid that will fit in the bottle; 35.4ml of liquid won't fit in a 30ml bottle naturally, so figure on having 25ml of liquid in a 30ml bottle (including flavourings/additives) before nicotine is added, then calculate your dosage based on that pre-nicotine volume to determine how much nicotine to add.
As for flavouring, there are recipes around, though some of them may not necessarily mention whose flavouring(s) they are using. For myself I've mainly been experimenting with all of my flavours to get a good general sense of relative strength of each so I have a better idea what proportions to use of each in whatever recipe I'm working on.
For example, I tried to make a cinnamon coconut cookie flavour using FlavourArt flavourings (Cookie, Coconut, and Cinnamon Ceylon). I presumed that the cinnamon would be pretty strong so I used the least amount of that, coming up with a recipe that was 60% cookie, 25% coconut, 15% cinnamon. Didn't work. Cookie turned out to be a lot stronger than I thought, as did the coconut, while cinnamon was weaker than I thought. So my next batch I'm going to try 45% cookie, 25% coconut, 30% cinnamon and see if that works.
It's pretty trial-and-error if you want to develop your own recipes, but you can try and follow some of the recipes on here or around the net and if it doesn't quite taste the way you want it you can tweak it by raising or lowering whatever ingredient you feel it needs more or less of.
Also, MTS Vape Wizard and AA Magic Mask, both from FlavourArt, can work wonders on specific kinds of juices to help mellow them out and bring suppressed flavours to the surface.