I have a table with columns and constraint

x varchar(250)
y varchar(250)
z varchar(40)
UNIQUE (x, y, z)

The charset is UTF8, so the key length is (250+250+40)*3 = 1620

With a page size of 16384, no problem. With a page size of 8192, I
get the following error:

key size exceeds implementation restriction for index "RDB$3"

RDB$3 is the (x, y, z) constraint. According to the release notes
(Firebird_v2.0.3.ReleaseNotes.pdf), the maximum key size should be 1/4
the page size. For 8192, this would be 2048. What am I missing?

Thanks!