How do you calculate interest for a 5330? Say it was due about a year ago?

DOL calculator is best place to do it.

The rates
http://www.dol.gov/ebsa/calculator/a2underpaymentrates.html

The calculator
http://www.dol.gov/ebsa/calculator/main.html

What do you do if somebody put in too much money instead of too little?

In other words, what about a nondeductible contribution? Say they did not catch the fact that they contribution was over the 404 limit till about a year later? How do you figure out that interest?

Say they made an excess contribution of $1,000, so that means $100 they should have paid a year ago. How do you calculate the interest on that if they do not pay it till a year later.