keep the 400 abut two feet away the first couple days ... expose the back of your hand to the light, to make sure it isn't too warm (optimal 75 degrees) ... if it seems to be handling the increased light OK, you can move it closer, generally, you will be 18"-30" away, if you have an aircooled hood or cooltube ... you answered your own question, the 'smaller' bulbs have smaller bases, the 'larger' ones have Mogul bases ... :smokin: