adjusting if/match formula to allow positive and negative movement

This is slightly difficult to explain without showing the file itself so I have attached an example of what I am trying to do.

I have this file with a bunch of items in row 2, some quantities associated with those items in row 3, an installation rate for each of them in row 4, and a lag between each activity in row 5. We have a timescale down the left in 30min increments (for the purposes of this, the timescale has no relevance anyway).

I have populated a formula in cells E6 to S208 which basically draws a programme for me based on the quantity and the installation rate of each activity, where by default the next activity starts as soon as the previuos one is completed, but the lag in row 5 can be used to put some space in between the activity and the one before it. e.g. if you look at activity 5, the lag of 2 has resulted in this activity starting 2 cells AFTER the previuos activity has finished.

The issue is related to this, my formula only works for positive lag, to put a delay between 2 activities, but what I also need it to do is be able to have negative lag, i.e. have the next activity start x rows BEFORE the end of the prevuios activity and my current formula falls down if I enter a negative number in row 5, it simply starts the activity from the end of the previous one.

Is it possible to adjust it slightly so that it works with both negative AND positive numbers in row 5? I can make it do one or the other, but have not been able to get it to do both within the same formula.

thanks

Yes just edited my post after playing around some more, it doesnt quite work yet.

My point was, it doesnt work. Using that formula with a negative lag of say -5 does not move the numbers to start 5 rows higher up than the finish of the previous activity, it will move it up to the same row as the previuos activity, but not 5 rows above it, even using -10 lag, it still puts it on the same row as the previuos activity.

thanks I think I have managed to make it work with that, probably should have spotted that myself, I was thinking about using offset somehow but couldnt get my head round it.