Thanks for the advice! So it sounds like if I want to find dice I'm [probably] better of NOT using a Rune of Wonder - have I understood that right?
Potentially, but it shouldn't matter all that much in the long-term. I just don't know for certain how a Rune of Wonder "Doubles the chance of finding Rare, Epic or Legendary items from chests". That's all back-end wizardry, and you can only guess at the math involved from the client-side.
Last we knew, the base drop rate to get a set of dice (any rarity) was about one in four chests redeemed.
If the RoW doubles the odds for every Rare/Epic/Legendary, then dice odds skew down to one in five because about half the dice are Uncommon rarity.
If the RoW doubles the odds for only Rare/Epic/Legendary cards, then the dice drop gets closer to one in six because all dice would be at a disadvantage.
Worst-case, the RoW could lower dice drop by 8-9%, but it is likely closer to 5%. Again, it's all best-guesses trying to sort out black-box stuff. The Treasure system is an arcane beast we may never truly know.