How do I edit a formula as part of a macro?

I'm trying to record a macro that will paste stuff from another work sheet,
then update the formulas by adding the items that are in the pasted part to
the ones that were already there. Whenever I try to do it the macro stops
recording until I get out of the formula.

You've reached the threshold that separates recording macros and
writing code from scratch. From here, to update a formula in a cell,
you'll have to know how to parse strings of text- basically taking the
formula apart letter by letter, and replacing old elements with new.
Can you give an example of the old formula, and what has to be updated?

I have a situation sort of like this one, but I have my macro set up to add a project. Down on the bottom there is the summary cells and they have the formula written in like this.

Cell K85 Formula = "=K7+K14+K21+K28+K35+K42+K49+K56+K63+K70"

Now say I add a new project, and that project is found on the next 7 set of cells, how would I go about adding K77 to the formula?

Link Strife