(I did ONE solo circuit before going CX solo - although I did a lot of dual NAV first).
Checking my log book I see that I did lots of solo circuits before any nav, but then only did two dual nav trips before going off on my own.
I did not, however, note the weather in my log book. My recollection is of several occurences of "the cloud base is too low for your first navex, why don't you fly some solo circuits".