After doing a bit more research on the barrows wiki page. It states that:
If players wish to receive more runes (which are generally worth more
coins) instead of bolt racks and key halves, they should aim for a
Reward Potential of 880. Having exactly 880 points maximises the
quantity of blood runes without having bolt racks take up reward
So this accounts for the max of 88%. But what about the 73%? Well, further down the page it states that the roll for blood runes:
Requires 756 reward potential to roll.
So 73-88 is essentially the best range to have to get the max amount of mind, chaos, and death runes, while also still guaranteeing a roll for potential blood runes. At 88%, you can receive the max amount of runes possible (if you are so lucky). After 88% (exactly 881 potential) you will roll for bolt racks and lose a potential reward slot for runes.
Just as a quick guide, these are the potentials needed to roll for each rune/bolt drop.
- Mind runes require 381 reward potential to roll.
- Chaos runes require 506 reward potential to roll.
- Death runes require 631 reward potential to roll.
- Blood runes require 756 reward potential to roll.
- Bolt racks require 881 reward potential to roll.
- Key halves require 1006 reward potential to roll.
- Dragon med helm requires 1012 reward potential to roll.
Note: The chance for a piece of barrows equipment is "unaffected by the rewards potential that is built by killing other monsters in the crypt."