Currently, the only way to directly assign a value to dynamic variable is to return it from a code node then map it to a variable. The problem arises from returning the value from the code node. That action adds the value to context. If then you insert the variable into the prompt, it gets added again - doubling your token count. If the code node is executed multiple time during a session, each time it runs, it inflates the context window and thereby the token count.