There is no problem with what youre doing.
if you would have been able to seed the tank, it just cuts down the time frame.
one thing- take readings before adding the ammo, then again after you add it and its had a little time to circulate around. If you always took a reading right after adding ammo, it'll never hit 0. Doing it before and after you'll be able to verify that you are adding the right ammount, and then you'll be able to see how much ammo your bacteria is breaking down over the course of the day (or half day, or whatever interval you are adding the ammo over).
The level of nitrite spike is dependant on a lot of factors. If you understand the process of two forms of bacteria, it can basically be looked at as how much faster did the ammo eating bacteria grow than the nitrITE eating bacteria grow.
6 days isn't (probably) enough to see a nitrite spike unless the tank was jumpstarted by other bacteria from another tank. And depending on the level of bacteria seeded, if you would have been lucky enough to have a source to seed your tank, you might not ever see a spike.