My replacement from SRB arrived yesterday. After some playing around with it I arrived at the following method, which seemed logical and worked for me. Start without filter fitted, frame shot, take a reading, work out exposure and adjust, focus, fit filter, shoot. I was using a 1DMkIII with a 24-105L fitted, along with a Canon RS-80N3 release, and thanks to the advice earlier in the thread I used the timer on the top LCD that is available when using the B setting. I was just shooting through an upstairs window at some trees in my garden with buildings behind, but the bright afternoon sun was good for using the filter to lengthen the exposure. I agree that the app I used is only a starting point for exposure, but it got me somewhere quite near and a little trial and error soon got me bang on. I did find that even with the filter fitted I was able to auto focus, which surprised me. Quite strange for a newbie to it have a 40 foot artic drive through an 8 sec exposure and not be in the shot! I'm looking forward to getting out down to the Wear, which I live @ 800m from, and along the banks into Durham for some shooting at the weekend.