Sorry I can't help you with the minutes per pound rule of thumb. I don't know the size of your Turkey, ducks or the temperature your cooking etc -- but logically the ducks are somewhat smaller and will/should/could take less time to reach 160*--- At BBQ temps (225*-250*) I would plan on 2 1/2 -3 1/2 hours + or -
If your final desired results are ducks with a crisp skin then you will need to kick the temperature up considerably to roasting temps...At least in the 300* + range. Pricking the skin (not the meat) with a meat fork in several places so the fat will render off will help the skin to crisp...Making small slits with a sharp knife will also work...You may want to place a pan under the duck(s) to catch the fat for other/later uses....
Also you may want to consider cooking the ducks for a couple of hours with the turkey -- laying on some smoke flavor...then take them in the house and finish off in a hot (400*-450*) oven. This may help your timing, and will certainly help with crisping up the skin...
If a crsip skin is not your objective then just cook with the Turkey to 160*...Tent with foil...rest for 10-15 minutes...Slice and serve.
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