I second Ed. You have done great.
Here's a rule of thumb I use that will help: If your OG is 1.075, take the 75 part and divide by 5. This give you your ideal pitch rate in millions of cells per ml (for you, 15 mil / ml)
NOw, there is a pitch rate calc at www.wyeastlabs.com
that will give you numbers on a 2 step starter. I find it indispensable. For you, the ideal would have been to use a 0.25 gal starter, then stepped it up again with 0.25 gal. If you did use a stir plate (or shook very often), you'd end up with 13+ mil cells.
The volumes you are
using will result in 8 mil cells which is not ideal for a 5gal batch. I must add, however, that I have under-pitched many times and it was still great beer.