If you look at this wonderful table by Shiraz
http://www.iceinspace.com.au/forum/a...e.php?a=217286
You can see that my gain was 80 so reference the 90 line , 30 seconds at f2 is equivalent to 6min at f7, so 13min would be 26 x 6, or 156min at f7 if I can extrapolate that?

The sky meter reading there was over 22