Doing a little math in a flow step using tokens

Hello all, 

I'm doing a little experiment to see if it's possible to calculate a new value in a currency field based on another currency field, using a flow step and tokens.  I know this type of thing works with date fields, for example {{lead.Date of Birth}} + 18 years, but am curious if this trick translates to other field types.  Here's what I'm trying to achieve:

• If someone has an Available Trip Credit under $1000, we want to double that and populate a separate field called Increased Trip Credit Value with the doubled amount.  So if they have a $400 credit, the newly calculated value would be $800.  
• We want to put a cap on the upper limit of that credit they can double (of $1000, so the most they can get is $2000).  

This is what I've attempted so far - Choice 1 works, but Choice 2 doesn't.  Not totally surprised about that...  Am I using the wrong formatting or anything, or is it just not possible?  Are there other ways to do calculations in Marketo besides this type of flow step and token trick?  

Thanks!