Thinning VG based juice (either 100%VG, or something less) is more of a personal preference than anything.
You will hear people say they like "thick" juice, but I find the carto materials like a somewhat narrow range of viscosity. Whether it's Karo syrup thinned with water, Quaker State 40 weight thinned with transmission fluid, or VG thinned with water or Ethanol, the filler or wick will move fluid based on viscosity.
I've even heard of vendors mixing to an exact "specific gravity", but that does not have a bearing on thickness (the thick 40 weight is lighter than water). But, measuring viscosity is not within the capability of the "Home Mixologist".
I base my mixes on the typical Dekang all PG juice. They have been making e-juice for a while and even though the flavor may change the thickness is pretty constant. When you buy VG juice you are at the mercy of the vendor as to what the thickness is. Just how thin did they make it? Is it diluted at all or is it syrup thick?
There is a way to "standardize" the thickness of your home cut PG/VG juice and have it come out the same every time.
Do a simple "drop test" with a juice that wicks just fine for your taste, then repeat the test for your mix. Thin as necessary to match the reference juice. Each time you mix up your home made juice, you have to let it sit until all the air bubbles has cleared or the results will be off. Might be a little time consuming at first, but it gets easier with practice.
The setup in the photo might be an overkill (two 10ml syringes with 16 ga needles would be fine) but you can compare the relative viscosity of liquids of about the same thickness with this test.