Sorry about my late answer.
Your formula is exactly what I was looking for, thank you very much!
I would just like to clarify a few points in case anyone stumbles upon this issue and finds this answer:
1. Because this is a WINDOW_SUM, for the calculation to work correctly, the steps must be precisely ordered in the visualization from Step 1 to Step N.
2. If you need to calculate percentage of projects starting from base 100% (Step 1 is 100% of total projects and decreases every step), you can modify Shinichiro's calculation to the following:
3. If you need to order your steps in rows, from top to bottom (as in Shinichiro's example, Step 1 is the top row and Step N is bottom row), you need to compute using Table (down). If you presented your steps as columns (Step 1 is the first column, Step N is the last column), compute using Table (across).
I hope anyone who sees this finds Shinichiro Murakami's answer as valuable as I did, thanks for your help again!