Hey @Alan_Home, this isn't possible currently using the "standard" Visualizations in Kibana. Implementing this would require two calls to Elasticsearch, one to calculate the 90th percentile and then another query to calculate the average, but excluding all values that are above this limit.
It's theoretically possible for you to create this yourself using Vega but it's a rather heavy lift. It'd be great to also capture this requirement in a feature request so we can address it more holistically.