I just finished browsing some recipes and it seems that making chai is anything but an exact science.
You can use just about any spice in any proportion cooked (steeped/brewed) for practically any length of time.
Of course there are the basics, which seem to be tea, cardamom, and ginger. Then add any of a thousand different items to taste and enjoy! It looks like experimentation is the only way to find your perfect chai recipe. Some interesting alternatives I've run across are chamomile, fennel, and peppermint.