[SalesForce] Assign selected picklist value as a default value to the Text field

Unfortunately, this will have to remain a manual effort as there is no way to get the time zone offset inside of a formula currently. I would advise trying to simply use the text field again and trying to get that to work rather than the formula field. It will make it easier for your users as well as properly display the time in their own time zone when accessing the system.

If you absolutely must continue to use this formula field, I would at least suggest using a custom setting instead of having -4 or -5 all over the place. I would create a custom setting for the current time zone offset. You can then access it like:

Formula Fields

Formula fields only work for hierarchy custom settings; they can’t be used for list custom settings.

{!$Setup.CustomSettingName__c.CustomFieldName__c}

Salesforce Docs: Accessing Custom Settings

Yea, use the FIND method coupled with an IF or two, and you should be on your way.

Example: (decimal) DesktopCount =

IF(CONTAINS(Product_Info__c,'DESKTOP:',VALUE(MID(Product_Info__c,FIND('DESKTOP:',Product_Info__c),FIND(',',Product_Info__c,FIND('DESKTOP:',Product_Info__c)))),0)

If the prefix text is there, grab the MID text between the prefix-text and its delimiter.

Update

I didn't test my function, but the basic idea is there. I want my formula to do the following:

(1)  Check if the "prefix-text" is there.  
       Example: is "Desktop:" in my string?
       Available Functions:  CONTAINS()

(2)  If yes, take the text between the prefix-text and the delimiter
       Example: prefix-text="Desktop:"; delimiter=",";  result = "10"
       Available Functions:  MID(), FIND()

(3)  If there is a result, convert it to a number
       Example: 10
       Available Functions: VALUE()

Here's a list of Formula Functions you can use.