Scenario is that while doing a multi-day CF, let's say , 121 hours at 36F with IDY at .13% based on chart (with no RT fermentation in this example) I decide that instead of baking at the original time of 121, that I need bake sooner Let's say 73 hours ..
If I'd known the new baking time when I mixed, I'd have used .26% IDY. So, assuming an RT exists of 64, can I drop down the chart in the .26% colum to 64F, see 7 hours, and add that to the 11 hours in the .13% column for that temperature, add them and get 18 hours at RT 64F to bring the dough to readiness?
No, if you start needing 121 hours at 36F and ferment the dough to the 0.26% IDY column, you still need 73 hours which means you have been fermenting for 121-73=48 hours. From there if you finish at 64F, you only need 7 more hours making only 55 total, not the 73 you need.
Remember that (theoretically) every time-temp combination in a given column takes you to a dough that is ready to use, so your 0.13% IDY dough should be ready to use in 121 hours at 36F (green circle) or 11 hours at 64F (pink circle) or any other time-temp combination in that column.
The simplest way to get to 73 hours, assuming you could easily control temperature, would be to just slide down the column until you find something close to 73 hours - ferment at ~41F (red circle), but that probably not a realistic solution for most folks.
Let's say your only temperature choices are 36F and 64F. You have to find a combination of (a) time at 64F and (b) remaining time at 36F that equals (or is close to) 73 hours or vice versa. That's a little easier said than done.
So let's say you mixed your dough and as you were about to put it into the fridge, your wife comes in and says "I hate to tell you this, but you need to bake in 73 hours - not 121 like I told you last night. All you have to work with is the fridge at 36F and the counter at 64F. What do you do? We have to find some combination of time at 36F that when added to the remaining time needed at 64F totals to ~73 hours. You have to work in the blue boxes.
Looking at the pattern across the 36F row, you see that if we go all the way to the right, we still have a dough that needs 62 hours at 36F. To get to that point, we have fermented 121 – 62 = 59 hours. If we drop down the column to the 64F row, we see that we need another 6 hours at 64F. 59 + 6 = 65 hours total; not long enough.
You can see that the 36F row drops by about 10 hours per column over at the right side, so you could guess the next column would be about 52 hours. 121 - 52 = 69 hours at 36F. Likewise, you can guess that the next column off the chart to the right on the 64F row is probably about 5 hours. 69 + 5 = 74 hours, so 69 hours at 36F + another 5 at 64 is pretty close to the 73 hour goal.
You could work it the other way too, doing the first step at 64F. In this case, the table tells us we need 11 hours at 64F. Let's say we do 4 hours at 64F. We would have 7 left at 64F. Slide across the 64F row to 7 hours (there are 2 of them) and then slide up the columns to the 36F row, and you can see that we would need somewhere between 80 and 73 additional hours at 36F. Add that to the 4 hours we did at 64F, and we see that we are between 84 and 77 total hours. A bit too long.
If we do 5 hours at 64F (on the 64F slide right from 11 to 6 hours), the table tells us we would need 62 hours at 36F which is 68 hours total. A bit too short, so the answer is somewhere between 4 and 5 hours at 64F which leaves about 67 hours at 36F for a total of about 72 hours total.
Remember just as the chart was only intended to help you find a starting point with the expectation that some experimenting and tweaking would be necessary, it can't be expected to deliver hour-resolution accuracy for this sort of thing either. I do think it's better than a wild ass guess, but you will need to watch it for the last 8-12 hours and adjust the temperature as necessary if things are progressing faster or slower than desired.
Example 1 below is 36F first. Example 2 below is 64F first.