Lie face down on a flat or slight incline bench holding a dumbbell in each hand with arms hanging straight. Keep a neutral spine and slight bend in the elbows. Lift the dumbbells out to the sides and up toward shoulder height by squeezing your shoulder blades together, then lower them back down with control. Repeat for reps.
Log your dumbbell lying rear delt row sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.