When a question is confusing, helpers need more information; it's not a matter of your doing something wrong. Here, your pictures are a little fuzzy,...
You haven't actually said how you are making the shelves out of rods (are they just the perimeter, and some other material fills in the middle?), or...
Comparing my spreadsheet with your data, I realized I was using the wrong balance for the interest calculation. Fixing that gives me more or less the...
Given that this is the question, what you did almost makes sense. But the numbers you averaged are not 25% for one room, and so on! You took visiting...