it seems that you take 25yr amortization for your mortgage which will require higher monthly payment. (which is what you can afford)
i suggest to take 35 yr amortization, to lower down the required amount for each payment. when you pay your mortgage, you still pay the amount for a 25yr amortization or more if you can afford, plus bi-weekly accelerate which will have 2 more payments each year to your mortgage.
when the rate raise, you will have some buffer between the required payment and your real monthly payment. if it shows a constant trend of raising rate, to a level that you're not comfortable with variable rate anymore, lock it in with a fixed rate then.