I used to sub KB SLDLs for back extensions, I think I used the 53#. That was plenty for me at the time.
If I still did CF, I'd now do 1/2 of the rx'd Back Extensions as reverse hypers (no weight added) and the other half as hip/back extensions with excellent form. I'd probably also be halving the rx'd amount off the bat to avoid overtraining and maintain form, but that's just me.
Jeff, you did get answers to the question you asked (# of DUs to substitute) and one you didn't (Steve and I both think that tuck jumps are a preferable exercise).
How to sub from the CF FAQ (I'm assuming you want these subs for CF WODs, so let's ask the experts on CF):
3.2. How does running compare to rowing, biking, etc.?
To compare the different aerobic exercises, pick a comparable time interval. For example, if you run 400m in 90 seconds, then row or bike or jump rope or run stairs etc. for 90 seconds.
3.3. What's a good substitute for running?
See previous for some suggestions. Also, box jumps, cross-country skiing, heavy bag work, kettlebell or dumbbell swings, weighted stair-climbing or box-stepping.
3.6. What if I don't have a rowing machine?
Do Sumo Deadlift High Pulls (see exercise page for demo):
If you don't have a rower substitute sumo-dealift high pulls (sdhp) with 45 pounds for men and 30 pounds for women. Take the bar from mid shin to under your chin--"shin to chin."
2K row = 200 sdhp's, 1K row = 100 sdhp's, 500 meter row = 50 sdhp's
3.12. I can't do double-unders, or I don't have a jumprope.
Do tuck jumps. Multiple single-unders in no way compensate for the exertion required for double-unders. Stand with your feet slightly inside the width of your shoulders. Bend your knees and lower your body down 8-12 inches. Explode into the air and bring your knees up to your chest in a tucked position. Upon landing, your feet should be in a strong, dorsi flexed or "toes up" position. Use your whole foot to generate power, not just your toes! Maintain good posture in your upper body. Keep your chest and head up. Don't let your shoulders lean out beyond your knees. This can stress your lower back. Explode off the ground as quickly as possible and repeat for the required number of repetitions.
1) Sub an exercise or number of repetitions that lasts about the same length of time.
2) CF has their own suggested substitute(s) for running, of which DUs are not listed.
3) CF has their own suggested substitute for rowing, SDHPs.
3) Tuck jumps are a CF-(and Garrett- and Steven-)approved substitute for DUs
To me, this would mean the best way to work in DUs (if you were insisting on using them, which would be fine by me) would be to pick a time interval and have the time intervals determine the length of your DU segment. Improvements in "fitness" (work capacity and coordination/agility) on the same workout scheme in the future could then be judged by an improvement in # of DUs completed in the set timeframe. Note that this may require two watches--one to time the entire workout, and one to time the DU intervals.
Hope that helps!