how do you mix up 51 and 48. Sounds more like you made up a time, figured out it was not the 4:20 pace you stated, did the math, and made up a new number.
in any case, it's an 800 workout. at 51, maybe you are 2:10 to 2:20
At 48.xx, maybe you are 2:05 to 2:15.
8x300 is actually a lot of volume at 800 pace. 5 or 6 reps can be sufficient, if you are really close to optimal time.