I've said in previous posts that you couldn't keep your own calculated measures in CTP1 as redeploying either the model site or rules and variables will destroy them. It would appear from Microsoft that the line for CTP2 is 'In CTP2, we will preserve customer changes to the AS DB by intelligently merging the changes.' Microsoft also went on to say about using native MDX rules in CTP1 to persist your own measures. Well last week I had already tried that and got all sorts of data access layer errors, but new day, new week, new brain 
and I tried again. We have lift off!
OK, what do you need to do?
First ensure that you have the Enable Native MDX/SQL Rules check box enabled in Planning Admin Console for your application.
Now check out your model in Business Modeler, got to the Business Rules tab and look for the Definition folder (grey background). Create a new rule to create your calculated measure in here (not sure it allows currentcube so explicitly define your cube)
Due to it being Native MDX, you can only use Definition Rule with Implementation type TemplateMDXScript.
Set the Status to Inactive prior to deployment as it won't let you save a native mdx rule straight away.
Then got into your application DB and find the RuleSetsOrRules table and find your new rule and set the IsActivated field to True.
Come back into the Business Modeler and refresh the model site, then redeploy your model site and your measure will appear (and stay this time).