I feed more than once but yes I add the nutes to a gallon jug with water then Ph then feed. I usually only water once a week though so if I only veg. for a month the plant only gets fed twice in veg. When the plant gets 3 weeks it should be ready, if it really needs food you will see the lower leaves start to yellow.
Depends on what kind of soiless mix you are using, I use Light Warrior on seedlings and just feed them water, they grow fine then when I do my first transplant I do a mix of Light Warrior and Ocean Mix, then I wait til week 3 or so and feed them some nutes.