Basement setting

[Al]

I just purchased a manufactured that's set on a basement and is sagging in the middle. Looking at the underside and using a 4' level all the support beams appear level without any bow to them. I will use a line level next to check the frame rails. I was wondering if there is a standard depth the support beams need to be set into the basement wall. What I think may be happening is the outside of the home is setting on the walls, but the rails had to "settle" down to the support beams causing the sag in the floor. Sound crazy? How can I find out what the distance NEEDS to be from the top of the basement wall to the top of the support beam in order to properly support the home??

[Eugene]

That distance needs to be the exact height of the main beams on the home.
I think your problem is somewhere else.
Is the center line (marraige line) of the home supported? every opening in the marriage wall 4' or larger needs a support under the floor. Generally at the ends of the home as well.
Are all 4 corners of the basement stair cut out supported?
Is the floor lagged together along the marriage line?
If the crossbeams are too low, they can be shimmed.