No it dont help. Its say the follwoing thing:

Msg 1035, Level 15, State 10, Line 28

Incorrect syntax near 'CAST', expected 'AS'.

He aspect a AS () something.

When i use that it

((CAST(SUM(nUsed_Min) AS FLOAT(2)) / CAST(SUM(nSize) AS FLOAT(2)) * CAST(AVG(100) AS NUMERIC(10, 2))) >= 80

No it say the following thing:

Msg 156, Level 15, State 1, Line 29

Incorrect syntax near the keyword 'group'.

When I use this syntax:

((CAST(SUM(nUsed_Min) AS FLOAT(2)) / CAST(SUM(nSize) AS FLOAT(2)) * CAST(AVG(100) AS NUMERIC(10, 2))) >= 80)

It says the following thing:

Msg 147, Level 15, State 1, Line 28

An aggregate may not appear in the WHERE clause unless it is in a subquery contained in a HAVING clause or a select list, and the column being aggregated is an outer reference.