I recently read a short document aimed at providing doctors with some general advice at switching a diabetic’s usual insulin due to a natural disaster like a hurricane. The general rule used is to switch to an insulin within the same general class, ie a rapid acting for a rapid acting or a long-acting for a long-acting. This tip-sheet then recommends starting with a 20% reduction in the replacement insulin dose and adjusting from there.
Your plan to use a 20% smaller dose is a safe one, I think. Good luck.
When I switched to Tresiba, I did it from an insulin pump. My total daily pump basal dose was 14 units and I ended up finding that 18-19 units of Tresiba worked for me.
Be aware that any change made to the Tresiba dose should be maintained for at least two days before making another dose change. Since Tresiba has a 42-hour action curve, it takes a few days to stabilize. This required some additional patience on my part but was worth it.