By default, Looker’s symmetric aggregate feature will return 6 digits after the decimal point of precision.
precision to be higher than 6 if you need more digits after the decimal point for your calculations.
precision to be lower than 6 when working with very large numbers, to avoid overflowing the decimal limit on your database.
For more information about symmetric aggregates, you can also see this Discourse article.
The following example creates a field called Average Value by averaging the Value dimension, and maintaining a precision of 10 digits following the decimal point: