I must laugh a little at myself here as I had my sign the wrong way ([Sales RT PCT] > 0.8) - otherwise I would’ve gotten back sooner. 
The below code will give you what you need.
VAR vCustomers =
ADDCOLUMNS(
SUMMARIZE( 'CPP POS', GROWER[CRM_PARENT_GROWER_MASTER_KEY] ),
"@Total Sales", [Total Sales],
"@Sales RT PCT", [Sales RT PCT]
)
RETURN
MINX( FILTER( vCustomers, [@Sales RT PCT] < 0.8 ), [@Total Sales] )
