No, you cannot used the alias that was generated on the same level on the SELECT
statement.
Here are the possible ways to accomplish.
Using the original formula:
select sum([some calculation]) as x,
sum([some other calculation]) as y,
sum([some calculation]) / sum([some other calculation]) as z
from tableName
or by using subquery:
SELECT x,
y,
x/y z
FROM
(
select sum([some calculation]) as x,
sum([some other calculation]) as y
from tableName
) s