L/R balance on rally Spd-sl pedals seems off (60-40).

Do a couple of hard efforts to allow them to settle in and then calibrate. 

Have you tried pedaling with left or right foot unclipped to confirm the pedals could register 100% left or right reading?

It is not a solution, it will show 100-0 or 0-100 if one of pedal under (or over)reports the power :) Could be 3 things, #1: wrong angle value in one of the pedals , #2 wrong crank length value in one of the pedals, #3 wrong slope calibration in one of the pedals .  I should remove the batteries , put them back, reset the install angles in head unit once more (but the battery removing is also do it), and do zero calibrate again. and tighten the pedals well @40nm . It will solve the #1, #2 issues,  if the #3 is the problem, You need to do static weight test calibration.

I am having this same issue. Left at about 35 and right at 65. This has bee pretty consistent over a couple of weeks. The left occasionally will read 0 and not report for several seconds at a time. The pedals are properly torqued.

Check that the battery doors and tightened down 

Thanks, I'll check that and go for a ride tomorrow and report back.

I've removed the batteries, deleted the pedals. Torqued the batteries to 5nm. Reconnected the pedals. The PP zones are in better alignment and now power is 45 left ant 55 right. I guess I need to loan the pedals to another rider to determine if I may actually be 10% + off balance. 

How is your saddle's height? Can You reach the pedals unclipped with your both heels continously without hip's moving? Can You pedal backward with this method? (So both heels on pedals and pedaling backward with stable hip position and there is no loss of pedal contact!). If not, your saddle is high, and this causes the imbalance so lower your saddle with 3mm and try again and again until it will be good. (The balance probably wont be 50-50 but if it will be 48-52 or 49-51 then that is half of success).

Good method checking the pedal's accuracy is the static weight test with a ~10kg weight, if You will get same numbers on both side then both pedals are reading equally (we dont need verify the accuracy of the pedals, we just need 2 numbers from both side and compare these)

After considerable playing with this I think it is time to call Garmin. In looking closely at the L/R power balance the left is frequently reporting 0 power. It had been showing a pretty stable 45/55. Yesterday for 40 miles it was 35/65. 

L/R image file

I then performed the static torque test. Initially the left pedal was way low, then it measured close to the right and then dropped out to 0. I did this numerous times with 24.96 lbs. The right never drops out. The left occasionally just goes away which I believe is the reason for the sizable L/R imbalance and is consistent with the L/R balance graphs from the eight or ten road rides I've done since receiving the pedals in early April.

Ah yes, if there are so many dropouts in the left pedal then it is obvious. If it is not contact issue (battery, battery cap or any parts in there) then maybe radio issue in that pedal. Call them