I can second the sentiment in this article about SLA/SLE being very hard to define. For example, my company originally agreed to an SLA with a customer regarding API response time. The problem was that the SLA was a flat 500ms and didn't take into account the nature of the possible queries. It's possible to request up to 18 months worth of data, which is never going to return to 500ms.

I had to spend 6 hours analyzing data to figure out what factors of a query actually impacted response time. It required me re-learning advanced spreadsheet skills to find correlations in log data. We're now in the process of rewriting the agreement because this analysis was not done at contract time.

This is a topic not really written about that much.