If you install six SSAS instances on same standalone or cluster server, you need to change default settings because when you start all of them at the same time, they all start with reserving 20% of the (physical?) memory.
When you start to run out of the virtual memory, you to change still more settings. Some of these settings can be altered on live and some require restart. It is safe enough to change these settings, but it is not easy to see if they help or not. Here are settings that at least didn’t do any harm on release 2017. As you note calculations can get quite complex, but the main idea is that the default value are bad for having lots of SSAS instances in the same environment.
Settings that you need to change have color coded background.