Kdub,

A good way to fix errors is to simply make a corrective entry for errors on previous pages. (Much like you might do for your checkbook).

Use parenthesis for subtractions, or add corrections as appropriate and annotate in the remarks section the date for which you are making the correction along with the reason that you are doing so.

It might look something like this:

3/7/08 1.0 Remarks: Added one hour to total time to correct calculation error made on 1/14/2003.

I'd suggest using a separate line for each error.