The lowest rep for all should be 15 then switch to another, never stopping. Just straight working out no breaks for at least 20minutes, if you can do more that's even better. As for the superman and 6 inches, the first goal is 1 minute, then adding 30seconds there after reaching the goal.
Another thing I like to do is for 1 minute see how many sit ups I can do. Then move on to a reps of maybe 15 Jack Knives followed by 15 V-Ups. Then see how many crunchies I can do in a minute. Turn it over and do 15 push ups. Then flip it over 15 - what have you.
It's really what you find comfortable/challenging enough for yourself. It's all about pushing yourself. Also you don't have to set the bar as low/high as I do. If you can only do 5, then that's great you know just keep working til that number gets higher! Or if you can do 15 and that's just a breeze, up the rep.
This is just for me personally but anytime I "up" anything it is by :30seconds or by 5's in reps.