Use it like this:

Select TOP 10 Device.nDeviceID,

Device.sDisplayName,

sgroupname,

sDescription,

dPollTime,

MIN(nUsed_Min),

MAX(nused_Max),

SUM(nSize),

(CAST(SUM(nUsed_Min) AS FLOAT(2)) / CAST(SUM(nSize) AS FLOAT(2)) * CAST(AVG(100) AS NUMERIC(10, 2))) AS Inuse

from statisticalDisk

where (CAST(SUM(nUsed_Min) AS FLOAT(2)) / CAST(SUM(nSize) AS FLOAT(2)) * CAST(AVG(100) >= 80