I did some calculation, so here's some insight on the best options if you're stacking Treasure Island chests over several events in order to reach certain amounts of spins in a future event:

  • 65 spins (~59 TI chest drops required) is the best option for Evolution Crystals.
  • 135 spins (~129 TI chests) is the best for Hero Crystals; or if you value both Hero Crystals and Evolution Crystals.
  • 35 spins (~29 TI chests) is obviously the best to get the most copies of Petrus; but it's the worst for Evolution and Hero Crystals.
  • 15 spins (~9 TI chests) isn't too bad for Evolution Crystals, and for Hero Crystals is better than 35 spins and in most cases probably better than 65 spins as well (depends how many events it takes you to gather 59 additional TI chests); however you'll never get Petrus.

Keep in mind that while stacking TI chests, you'll still get 5 Hero Crystals each event only using the daily free spins. Which means that if you value Hero Crystals a lot (more than Evolution Crystals), unless you can reach 65 spins in one event, stacking TI chests until you can reach 135 spins is definitely the best option.