Decrease evenly across row

I am working on 3 x 3 ribbing on a sweater.

I have 83 stitches and pattern states to decrease 23 stitches evenly across the row. How do I go about figuring when to do this? First and last stitch of row are knits (selvedge edge).

What happens after the decreases? Do you continue in ribbing or do you switch to stockinette?

If you switch to another stitch pattern, then you can just divide the decreases as evenly as possible. If you have to continue in ribbing, it will get more complicated.

http://www.thedietdiary.com/knittingfiend/tools/EvenlySpace.html

This has a calculator to decrease evenly.