My current 800m reps sessions are aimed for a 27:30 5 mile run, it doesnt mean it will happen but this is my target.
If you are planning a sub 27min 5 mile then if you run 800m reps you should be aiming for 25:30min pace 800m reps, this gives you a target of 2:32mins per 800m with 200m recovery which you could jog or walk.
If you increase the reps distance in line with the paceas you progress, then even if the 25:30 time does not happen then the sub 27mins and faster will.
Best Wishes, NN