Use CROSS APPLY
SELECT
pgwName AS [Part Name],
active AS [Active],
boardName AS [Vinyl Board],
rollWidth AS [Roll Width],
[Glass Height]
FROM
parts
LEFT JOIN cutVinyl ON parts.vinylBoard = cutVinyl.boardName
LEFT JOIN rawGlass ON parts.outerSku = rawGlass.sku
CROSS APPLY (
SELECT CASE WHEN blockHeight > blockWidth THEN blockWidth
ELSE blockHeight
END AS [Glass Height]
) AS CA1
WHERE
(
(stretchRadius IS NOT NULL OR stretchRadius = 0) AND
rollWidth < [Glass Height])
) ...