Not that I doubt you but can I please trouble you for a reference so
that I can get a precise estimate and which I can also use in the future
for forthcoming Berkeley clusters?

Anthony,

My source is Deep Sky Field Guide to Uranometria 2000.0
where on plate 97 it plainly saith:

05 20.6 + 30 36 Be 17 13'diam 100* 16mag photographic
moderately rich, small brightness range, fairly even
distribution,
detached

Which is a little disconcerting because it should have appeared
as a proper little smudge like Tombaugh 5. But it didn't.
That means I was seeing something else and Be 17 is going
to be another tough object and I may not be able to see it at all.

Ben