Homebrew PCBs

Step 8 is a good exposure point for negative films, I don't know about 
positives.  Based on the photo, I'd shoot for an exposure time of around 
85 seconds - your step 1 (340 seconds) is overexposed, but by step 8 (21 
sec) you're underexposed.

Note that "step held" is the highest step that's still getting barely 
enough light to do something, so in your picture you're holding 
approximately step 12 - the highest step that still shows any trace of 
your inkjet pattern.  Correcting for that also results in an exposure of 
around 85 seconds.

When using a step wedge, you don't always go by which step has the best 
*pattern*, but usually by which step has *anything*.