Prey size

I have a 390g ball from I read I should feed a 39g mice or rat? I feed it every 7 days

It will be harder to find 40 gram mice then rats. I would feed every 5 days.

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x2. At that size, a 40 gram rat (or 2 20 gram mice if they won't take a rat) every 5 days.

So I'm on the right track for size?

yes the right track but the low end.

10% body weight is the starting point with 15% being the higher end.
so 39 grams upto 58.5 grams.
if you are feeding on a 7 day I'd lean to the heavy side for sure, on a 5 unless the snake is building eggs or recovering some weight then maybe the lower end to middle.

I guess I got to start buying small rat. :)

10-15% of their weight is a good way to judge if you aren't sure. I personally just pick something that is just slightly thicker than the snake.

Here is an example I took to share, I think he was roughly 80 grams in this picture. He has that size of prey in his belly in this picture.
http://i157.photobucket.com/albums/t...ostmojo018.jpg

I just wanted to say thank everyone for your help. I fed my 390g ball first weaner rat it was 31g and the Snake didn't waste any time as soon I drop the rat in the tub BAM the snake got it.:D