I've realized since I started writing this tutorial just how "trial-and-error" this variable quilt is. It's hard to give concrete instructions like I would for a "cut-em-all-the-same-size" kind of t-shirt quilt. But onward I go. If it helps someone out, that's all I'm looking for...
So, as I continue to add borders to each block, I work from the top left corner down so I can see how I'm doing on length. It's very important to keep an imaginary 3" border around each block so you can make sure one column is not super long leaving another column super short. The remedy for this? Move your blocks around. It's a giant puzzle and I didn't stop moving blocks around until I had 3 or so left to add borders to. You could establish the length with one row, but I hesitate to do that with the fear that I'll need to move a block and then it won't work out any more.
At the same time, you have to worry about width. Notice how much more narrow the "11" block is than the one above it. The way to fix this is to increase your border fabric to 4" or more if necessary - measure!
You can see in this picture that there is a complete row of bordered blocks on the top row. This is important to make sure you keep your established width. (If you're very observant, you can also see that I've moved blocks around!)
As I try to keep the width consistent, I also move blocks that "fit" better in one row than another. I've ended up with a row for most of the wider blocks so they fit together well. I also have a narrow row. It's easier to group as many as possible like this so you don't have to add tons of border or take a bunch off.
Here's an instance where it would be easier to trim a little off the left and right sides of that red block than to measure and figure out how to add the right amount of border, or to trim it off later and be left with a 1" border which would look funny.
I had hoped to start sewing blocks together within columns, square those up, then sew the columns together but my leaky iron (Rowenta...) has stopped heating and it's replacement is not due from Fedex until tomorrow, so I must hold off.