As long as you are w/out expectations, then the letter is great! It has often been said to use writing letters to purge your feelings and get them out, but that advice also says not to send it.
However, in your case, I do agree w/Kerry that I don't think it would have been good for you to not have sent it. For you, the therapy is in getting it out there and presenting your stbx w/the "opportunity" to read it or not read it.
You said you didn't care if he read it, so I'm also glad you hit the send button. Again, this exercise is for your benefit and not his.
I'm glad you are taking these steps as they are so necessary for your long-term survival and recovery.
It will be good to see the kids today. As for stbx, just smile and wave.