I second what Steph was saying. I've seen a lot more attention to detail from my dc since starting dictation. They are more careful in their writing and better at proofreading their work.
The way we have been doing dictation is I go over the passage with them while they are looking at it. I point out any unusual spellings and for the sake of my son I usually review a phonics rule or two that is displayed in the passage. I point out punctuation and capitalization and any homophones that might be present. I have them really study it for a few minutes. Then they close their books that have the passage, and open their composition books. I say the first sentence or phrase, they repeat it to me, then write it. We repeat this until we have completed the passage. I then tell them to check over their work. They proofread it, then check it against their master list. If they got any mistakes, they mark that in red on their master sheet and make the corrections in their composition book in regular pencil. If they got everything correct, then they check it off the master list. I typically read it while they are writing to make sure that I agree that they have done a good job proofreading. If I haven't been able to check it while they work, I take a quick look at it before they check off the master list just to make sure they have indeed gotten everything correct. This whole process takes us less than 10 minutes each day and my dc really like dictation.
My dd is a natural speller so she has had no problems whatsoever with any of the spelling in the dictation passages. She can be careless at times, so dictation has helped her to slow down, be more careful, and proofread her work better. For my ds who is a very visual learner, it is helping him in his spelling so much. I have seen him during dictation struggle with a word and I'll ask him to visualize the way it was on the list. Most of the time, he then gets it correct. I've also seen him in other writing, stop and say, "That doesn't look right." I think that is coming from the careful attention to detail and visualizing that he is getting with dictation. I can't give you a grade level or anything else that he has improved yet. We test next month so I'm excited to see how he has improved. Last year, he was right at grade level with spelling. It was his weakest subject. I really think we are going to see some improvement in that this year due to dictation.
Here is a link that talks about dictation success:
viewtopic.php?f=6&t=6895&p=50262
Here's another link that talks about the benefits of dictation:
viewtopic.php?f=6&t=6835&p=49846
I don't know if that helps or not. I'm not for sure how you evaluate except to perhaps notice overall better spelling and attention to detail in all our your ds work. Hopefully some of those links will be helpful.
He tends to be a fairly natural speller (not all of my older kids have been).