heres a conversion+formula link this might help with units Conversions & Formulas
i agree with all of your suggestions,
5 mililiter to 1 gal seems normal
read your directions what ever it recommends make a 1/4th mix solution
USE 5ml (about 1 level teaspoon) in 10 litres (2 gallons), as your half strength dilution. I use metric as well so I know your dilemma. Your seedlings are looking quite healthy, and I endorse the light dilution at first with plants so young. If you can get some sea tea (pure kelp) it will help boost your babies along. Good luck mate.
Get your ph tester and all your nutrients. Test the water you feed the plants with and note the pH.
Add A VERY small amount of each nutrient to the test vial (one at a time), or mix it in a batch, then pour a small amount in the tube, and see if the pH goes up or down, and how much.
You can also test bottled water to see if you will be better off with that.
Now you can control your ph by the brand of nutrients as well as monitoring the soil.
So with nutes you will be adding 6.6 pH to the soil, a good base to work with when managing the nutrients. I think you will finf the 15-30-15 will lower you at least 1 lower so it should give near neutral pH if you use it by itself during flowering.
Today Baby1 entered 11th day Baby1 I to start to use 5ml nutes and 1ml
ph lower in 4 gallon water today only then completely to let the PH
value fall 6.2
I only then felt relieved drinks to Baby1