When to switch seedlings from fluorescent bulb to 400 watt Mh?

I started some seeds a little mini green house thing, the other day they sprouted up. Today I took the two tallest, still only about 2 inches, and put them under my 400 watt MH bulb because they were getting to big for the mini greenhouse. Now my question is did I move them to soon? Should I keep them under the fluorescent lights for 24 hours then when there ready for 18/6 put them under the MH bulb? Or just keep them under the MH bulb for 24 hours then eventually switch to 18/6? Not really sure when sure supposed to make the transition from 24 hours of light to 18/6 and also how many hours of darkness there really supposed to have? Thanks.
P.S. one more thing the ballast on my light has something on it that says dimming options, which I assume is the power output, then it says 50% 75% and 100%. I have it on 50% right now because I was really sure where to put because my light didn't come with a manual. Again, thanks!

Its never "too early" to switch to HID lighting. You can run from seed to harvest under HID if you like.

The reason why many people run seedlings under fluorescents is just because it uses less space and energy. There is no point running a full 400 Watt setup (with fans, etc) just to light a few teensy seedlings.

Its perfectly fine to run plants vegetatively 24-0, if you like. While some may disagree, its my opinion that vegetative plants don't "need" a dark period. If you prefer 20-0 or 18-6 for whatever reason, you can make the switch any time you like.

The dimming option on your ballast probably does just that. . .reduces the power output from 400 to 300 or 200 watts. Less energy = lower bill, but also less light. You might consider running the dimmer when your plants are small, and don't need the full power output of your lamp.

Also in terms of type of lighting, FYI, you might check this out. There is definitely a school of thought saying that MH lighting adds nothing to a grow. I can say for sure that you can get top quality buds running HPS from seed to harvest, if you so choose:

http://www.cannabisculture.com/v2/articles/2611.html

Thanks for the info, but FYI Im using the MH for the beginning stages then switching to HPS for the flowering stage, I've read it's supposed to be best.

i guess you could switch whenever you really want, i've grown from seedling to harvest under a 400w HPS.