ترکیب as good as (به خوبی، همچنین) دقیقا معنی not only و but also (نه تنها، بلکه …) را می دهد.

Your guess is as good as mine

.Def : We use this expression to say that we don’t know something

For example :

؟What time does the film end
؟Your guess is as good as mine. Perhaps around ten

؟What do you think we’ll be doing next week
.I don’t know. Your guess is as good as mine. They never tell us in advance here

He’s clever as well as nice. (= He’s not only nice, but also clever.)