SSAS - what could be a good starting point for memory settings? I have received the following alert regarding memory settings of SSAS:

Alert: SSAS 2014: Invalid TotalMemoryLimit Configuration Source:
MSSQLSERVER Path: SASBI01Alert time: 9/15/2015 5:38:50 PM
Alert description: The configured TotalMemoryLimit for the SSAS instance (307.1728515625 GB) exceeds the configured threshold required for necessary operating system functions and may lead to system instability or failure.
Total memory on the server: 0 GB Total memory
usage: 0 GB Memory used by non-SSAS processes: -3.89069747924805 GB
Memory used by SSAS: 3.89069747924805 GB SSAS Memory Limit Low:
249.57795715332 GB (0%) SSAS Memory Limit High: 307.1728515625 GB (0%)

What could be a good starting point for those memory settings in SSAS? Please see the image below.

This server has ONLY SSAS running on it.
Depending on how much RAM is on your server, below is a good starting point (given you have SSAS ONLY running on the server) :
click to enlarge:

Refer to :SSAS Memory Configurations for Common Architectures and Analysis Services Memory Limits

SSAS uses memory limit settings to determine how it allocates and manages its internal memory.  Memory\LowMemoryLimit defaults to 65% of the total available physical memory on the machine (75% on AS2005), and Memory\TotalMemoryLimit (also sometimes called the High Memory Limit) defaults to 80%.  This is the total amount of memory that the SSAS process itself (msmdsrv.exe) can consume.
Once memory usage hits the Low limit, memory cleaner threads will kick in and start moving data out of memory in a relatively non-aggressive fashion.  If memory hits the Total limit, the cleaner goes into crisis mode… it spawns additional threads and gets much more aggressive about memory cleanup, and this can dramatically impact performance.