Have you tested out the pattern in a fabric similar to the shirt you cut up? and do you have any excess fabric besides the 3 and 1/2 yards mentioned?
If not, go get practice fabric that is similar to what you will be using
and make sure it's similar to the shirt you cut up...
I'm assuming that the shirt you trimmed up was a generally made "T-shirt" which is a knit fabric and if the fabric you have 3 and 1/2 yards of isn't knit, it may not fit the same way
any time I make a pattern from scratch...and since I make most of my patterns from scratch...
I do a test, or a sample, where I buy maybe the same amount for the finished project as I do of the practice fabric. This enables you to practice cutting it out, and sewing it together. seeing where it fits, how it fits and letting you know if the pattern you made will work how you envision it....
hope this helps! ^_^
Katsucon 2014, Otakon 2014