Since I don't know what Excel 10 is, I'll give you a method that will work with Excel as far back as I can remember.

Look at VLOOKUP with the TRUE option. The value you are looking up ON should be sorted lowest to highest. The way VLOOKUP works with the True option is that if it finds an exact match, it uses the value in whatever column you selected. If it does not match it "falls up" to the next lowest value in the searched column and returns the value of the column you want to return.

VLOOKUP is a powerful tool when working with stepped functions.