You could always try something like
SELECT
SUM(
CASE
WHEN VoteTypeId = 2 AND Posts.PostTypeId = 1
THEN 1
ELSE 0
END
) QuestionsUp,
SUM(
CASE
WHEN VoteTypeId = 3 AND Posts.PostTypeId = 1
THEN 1
ELSE 0
END
) QuestionsDown,
SUM(
CASE
WHEN VoteTypeId = 2 AND Posts.PostTypeId = 2
THEN 1
ELSE 0
END
) AnswersUp,
SUM(
CASE
WHEN VoteTypeId = 3 AND Posts.PostTypeId = 2
THEN 1
ELSE 0
END
) AnswersDown
FROM Tags
INNER JOIN PostTags ON PostTags.TagId = Tags.id
INNER JOIN Posts ON Posts.ParentId = PostTags.PostId
INNER JOIN Votes ON Votes.PostId = Posts.Id
WHERE
Posts.OwnerUserId = @UserId and
Tags.TagName = @Tag
EDIT:
You could use a CTE and then used the columns in your calcs.
Something like
;WITH Vals AS (
SELECT
SUM(
CASE
WHEN VoteTypeId = 2 AND Posts.PostTypeId = 1
THEN 1
ELSE 0
END
) QuestionsUp,
SUM(
CASE
WHEN VoteTypeId = 3 AND Posts.PostTypeId = 1
THEN 1
ELSE 0
END
) QuestionsDown,
SUM(
CASE
WHEN VoteTypeId = 2 AND Posts.PostTypeId = 2
THEN 1
ELSE 0
END
) AnswersUp,
SUM(
CASE
WHEN VoteTypeId = 3 AND Posts.PostTypeId = 2
THEN 1
ELSE 0
END
) AnswersDown
FROM Tags
INNER JOIN PostTags ON PostTags.TagId = Tags.id
INNER JOIN Posts ON Posts.ParentId = PostTags.PostId
INNER JOIN Votes ON Votes.PostId = Posts.Id
WHERE
Posts.OwnerUserId = @UserId and
Tags.TagName = @Tag
)
SELECT QuestionsUp * 5 +
AnswersUp * 10 +
(QuestionsDown + AnswersDown) * -2
FROM Vals