Calculate the starting transferred balance. Add the transfer fee and any upfront cost.
Count the promotional months. Use the actual expiry date, not a rounded “12-month” assumption.
Set the required monthly payoff. Divide the balance by available months and add any interest under the promotion.
Keep new spending separate. Check whether purchases earn a grace period while a transfer balance remains.
Model the failure case. Calculate what happens if part of the balance remains when the standard rate starts.
Decision rule: do not transfer the balance unless the monthly payoff amount fits your budget without relying on another transfer later.