That's what I was afraid of. I will try to explain this as clearly as possible and step by step, but it's not easy. Don't hesitate to ask and repeat questions if necessary.
The green boxes are the ones that correspond to your specific case and that you can modify. You should only read the others: they allow you to find the correct value for the commands you will use. The difficulty is to find the right box in each situation.
This time, I'm going to take a completely different approach to help you find the values for the two green boxes that correspond to your case.
To begin, you define the area that will be covered by the roof with four walls. On these walls, you place slope blocks whose diagonal corresponds to the roof slope. You can use the height of these slope blocks as the variable "h" in the spreadsheet, and their width as the variable "m".
Next, you'll place a tall, thin block against these slope blocks to form your roof, tilting it so that it's aligned diagonally with the slope block. To do this, you need to know the dimensions of this roof block and its angle of inclination.
Now, we need a numerical example. Let's say your slope block, at the top of the wall, has dimensions X4 Y3 Z1. You can use its height Y=3 as the "h" variable in the spreadsheet, and its width (or thickness) Z=1 as the "m" variable.
For your roof block, the simplest approach is to keep the same width X=4, arbitrarily choose a thickness, for example Z=0.125, and find the correct value for the height in the spreadsheet: it's in the "Straight slope length" table. Here, we'll choose the value for x1 (Y=3.162278), but any value in this table will work, depending on the situation. All that's left to do is type the command "size 4 3.162278 0.125" to get the roof block the correct size.
Now you need to position and tilt this block at the correct angle. This angle is found in the "Angles" table, in one of the two "Straight" columns. To find the correct value more easily, the simplest method is to roughly tilt the roof block in-game, read the value IG, and find the closest match in the table. Here, IG, we note that the angle is between 15 and 20°. The closest value in the table is 18.434949, so we can type the command "rotation 18.434949 0 0" and position the block.
The rest is simply a matter of repeating this procedure, copying blocks with INSERT, resizing them with the arrow keys, positioning them correctly using modular snapping… and once you've mastered the procedure and its underlying logic, you'll quickly end up with a complete roof.
There are many other little tricks to correct apparent flaws or build more efficiently, but I'll answer those as questions arise. This post is already quite long.
P.S. The blueprint is an example; the lower part was created using the values provided in this post.