|Where do you enter the value for heave and slack?|
If I were doing an excel sheet, I assume you date and time stamp each heave and slack?
a1 = heave or slack
a2 = amount
I would use an if statement. Look at a1, if heave, then cell = currentamountcell-a2, else cell = currentamountcell+a2.
Use a reference to the cell versus the absolute. :) You can then drag the formula down the sheet and it will repeat the pattern.
You could do this all in vba of course, but it depends on how complicated you need to make it or how you are placing the adjustments to the chain length.
Provide a little more sheet design information, and you may find a better resolution. :)
I'm still learning everyday.