Points to consider, in my view:
The average 100m split for a 49.00 second 400m performance is 12.25.
It is reasonable to assume that an athlete capable of such a performance is able to run 100m from a rolling start in 11.0 seconds in a single set of 10 reps with appropriate recovery. As much fast relaxed running at a pace aound or quicker than 12.25 per 100m, particularly in a climate of endurance development would seem to me to offer enormous benefits to the one lap sprinter.
The optimum range for Intensive Tempo work is around 85-90% effort. This range equates to between 12.95 (85%) and 12.22 (90%) for 100m reps.
Duration of recovery will depend on the training status of the athlete(s) concerned.
I prefer experienced athletes to take 60 seconds recovery per 100m worked in such situations, but I think DMA’s suggestion of extending this (my own suggestion would be 75 sec per 100m worked) has good foundation, particularly in developing athletes, depending on their level of maturation.
If you are using grass to work on, however, (you didn’t specify) I would say your 12.75 target is a good one.
The primary focus of all Intensive Tempo work should, of course, be on a sound technical model.