Step 3: Do the Math
You don't need a sixth sense of how much stuff will fit in one space. A lot of it is simple mathematics. It's simple—you just need to calculate how many items will fit into a given space.
Here's how you do the math for any space. First, measure your shelving space or hanging space figure out how many items can fit in that space without being crammed.
Then, sort your belongings until you get to a number you know will comfortably fit into the space you have. If you're feeling ambitious, get rid of even more so you'll have room to grow.
As you draw up your plan for the rooms, let math be your guide. What's physically possible for the space? Measure the total length of your bookshelves. How much linear footage for books do you actually have? How many books will fit there? What is the hanging space you have available? Assess how many items will hang in that space so you know what to discard. You need to assess your space limitations and design accordingly. This will also help take some of the emotion out of the disccusion—you only have the space you have!