I understand you are asking about the annual fee and not the interest. Two of us have now responded that it would be best to have two separate cards - one for business and one for personal as it is a goo business practice not to comingle the two, esp. if the business expenses are a separate company.
we are both stating it's a 'slippery slope' to have a well documented approach on how you would split that annual fee. Dollar value of all transactions for the year? Number of transactions for the year? etc. Might be best to not deduct it in 2018 and get yourself a separate card for 2019.
again, if you are employed by someone else, these are unreimbursed business expenses and not deductible in any event
