If using your small-small method, I'd carefully check the big chainring - biggest cog combo before riding it. The chain needs to be long enough to not jam when shifting into the big-big.

I sized using the usual "big-big + 1 inch" method, and it was fine. Like previous comments mentioned, this might be best if you are exceeding the recommended cassette sizes.

A rider might never want to use the big-big, especially on a triple, but it's easy to lose track of the current gear, and shift to the big-big by accident.