I'm curious what I should have done.  It's pretty doubtful that I was efficient on this one...

I have a Contact as a lookup field on an Opportunity. What I needed was a button for the User to select that would add the related Contact's mailing address (street/city/st/zip) to two different sets of address fields on the Opportunity (essentially a shipping/billing).

What I did was a detail page button with a content source of URL.  

I couldn't figure out how to directly call the SomeContact__r.MailingStreet  field so I created so I created four formula fields on the Opportunity that each get those SomeContact__r fields. 

What I ended up with was a URL with 8 instances of fields being merged in the URL as follows:

https://na4.salesforce.com/{!Opportunity.Id}/e?retURL=%2F{!Opportunity.Id}&

SomeFieldID={!Opportunity.FormulaFieldToContactInfo__c}&....

&save=1

I'm assuming that URLs with Street in them could break in the future and I think I have to leave the Opportunity address fields on the page layout. 

Assuming this isn't the smart way to do it, what should I have done? 

Thank you.