There are three account needed to install Service Based Architecture and the Web Client. The first is 'sa'. If you're giving the chance in the install to use 'sa', you should do it.
Second is the domain service account. For the purposes of this article, we'll assume the same account for all the services. The install asks for domain/username/pass, obviously you can't put 'sa' here so you'll use the service account that you already created for this purpose.
Last is the SQL account that will be used to communicate from the web service to SQL. This is not at all clear, and it needs to be keyed in two places.
Look here, at the 5th screen shot entitled 'GP Configuration'.
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This is a SQL User Account that will be created on-the-fly. Carefully note that it is not the domain service account but a SQL account.
Now look here, and scroll to the bottom to the window entitled 'Additional Tasks'. This form is accessed via Dynamics Utilities. To solve the issue raised above we needed to use the same SQL User Account that we used in the steps above. We had done a re-install and used a different account.