Plain water will ride you out until you are ready to transplant out of cups or trays. If you are using a sterile medium, I would introduce your nutrients at like you said, 1/4 dosage. If they start yellowing like they are needing more nutrients, give it a half dose, so on, yada yada. My Apollo 13 plants are in a 1:1 ratio of Pro-mix and Happy Frog. After the first watering with no nutrients they were yellowing like Nitrogen Def. The next time I watered a day or two later, I added the week 1 dosage of nutes and they blew up.
Typically though if I use any soil in the mix, I just feed plain water and let it eat the nutes in the soil and then once it shows me it wants more nutes, I move to week 1.