You could use a higher N, but I think 15-5-5 sounds pretty good.

To make a diluted solution, you can measure out a teaspoon or whatever onto a table, and keep halving the pile until you have an 1/8 or whatever fraction you want to use.

Or, you can add water to dilute a solution. If you make a gallon solution, and want a 1/4 strength, you add 3 more gallons.

I wouldn't suggest spraying ferts on the leaves. The leaves dont function to absorb nutrients, roots do. Plus the ferts could burn the leaves if the lights are on. I liked to spray the leaves with water to cool them down and keep humidity up.

Once you begin flowering, don't spray the plants at all. You want to keep humidity as low as possible.

hope this helped.