Interest Rate Hikes – positive gearing %

What percentage of your “positive gearing” margin should you allow for a hike in interest rates? If you locked in at say 5% for 5 years and when you have to then pay interest rates of the time(after the 5 year period), this could be 8/9% (also allowing for an increase in rent). How much of a hike do you allow for in your current positive gearing calculations? If you are only making say $20-30 per week as slight hike in interest rates will wipe out your margin[?]