Pairing Rats, Questions?

Ok so this is my first time trying to breed rats the first time I had a litter was on accident( I left a male and a female in a cage until next feeding and the female was pregnant).

Here is what I am doing:
-I have 3 bins with 2 female each
-And 1 bin with 1 female..
-I only have one male at a time and I am swaping him around the cages like i would do a male ball python (4 days in each bin)
-Its been about 3 weeks

So now my questions:
-Should I leave the male in longer?
-Does the male have to stay in the tub the whole time?
-How long is the gestation?
-Can I keep the male in with the babies?
-Is there a definitive way to tell if a rat is pregnant?

Thanks for all your help

Rat set up (the fish tank is no longer there)

http://i1108.photobucket.com/albums/...r/DSC_0037.jpg

You can keep the male in with the babies most of the time. I'm going to say most of the time because I have run across a few males that were very aggressive. I haven't seen any of mine being mean with the babies because if I see one that is rough on the female breeders they become snake food.

My male rats stay with the females all of the time. Their attitude toward the babies ranges from complete indifference to curiosity or avoidance. I might go to swapping between two cages in the future. I wouldn't try swapping after 4 days or you might miss some of the female's heats as they are 4-5 days.