This is discussed in more detail here, complete with how the percentages map:
Compared to the PowerWall reading with our Tesla app, the Gateway's local website is consistently about 4% higher. The Tesla app appears to be correct - with the reading around the value set for our reserve power. Is anyone else seeing this? Is there any fix to get the two readings to match?
teslamotorsclub.com
(Tesla Pros uses the same data as the gateway API.)