I went out again and got another 40 minutes of exposure for M51. This time I was much more diligent about flats and darks.
I ended up stacking tonights data and the data from 5-28-16 to give ~80 minutes of integration time. DSS makes doing multi day stacks pretty easy as long as you’re saving all of your calibration frames and data!
40 extra minutes of integration only gave a little bit more detail, but it helped separate the signal from the background so it was a lot easier to add contrast between the sky and the target!