I think I have found an efficient way to craft high-level runes. Comments welcome.
If you want to craft a level-2 rune, you need four level-1 runes. Outcome is 100% certain. Therefore, the value of a level-2 rune is four level-1 runes.
The value of a level-3 rune is four Level-2 runes, in other words, sixteen level-1 runes.
Generally, the value of a level-N rune is 4^(N-1) level-1 runes. [4 to the power of N-1]
That means, a level-10 rune takes 4^9 level-1 runes to craft, that's a quarter million!
But we get runes from mazes, dungeons, and so on, and they are not all level-1. So it is not as bad as it seems.
When "flipping runes", we use 3 level-1 runes of the desired type, say speed, and one rune of high level, let's say level-5 team stamina. We hope to get a level-5 speed rune. When we craft it, the game tells us that the chance of getting a level 6 rune is 25%. Experience tells me that the normal outcome is a level-5 speed rune, and occasionally we'll get a level-4 rune.
I assume that the chance of a level-4 rune is also 25%, and that in 50% of the cases the rune will be of the same level as the highest input, in this case, level-5. If anyone can confirm or deny this, please do.
To get to the point, this probability of crafting a rune a level higher than the original one, opens a clever method to accelerate your way to level-10 runes. This is how it goes:
Never, ever, craft a level-2 from four level-1's.
Always craft three level-1's with the best rune you possess.
Your existing level 2, 3, 4,... runes you can raise the old-fashioned way.
Here's why: Assume you have four level-8 runes of the same type. Each of them is worth 4^7=16384 level-1 runes, to a total of 65536 level-1 runes.
If you use the four runes to craft a level-9 rune, the four level-8 runes will disappear. You will remain with a level-9 rune worth 4^8=65536 level-1 runes. You gained nothing, you lost nothing.
But if you use 4x3 level-1 runes, trying to flip the level-8 runes, this is what happens: Statistically, in one case you will get a level-9 rune. In another case you'll get a level-7 rune, and in the remaining two cases you'll get level-8 runes.
That means, you have your level-9 rune as before (65536) but the level-8 runes didn't disappear: Two of them you still have (2x16384=+32768), and you have an additional level-7 rune (+8192). For this to happen, you destroyed 12 level-1 runes (-12).
In other words: The gain of getting a rune of a higher level (65536-16384-3=Gain:49149) outweighs the loss of getting a rune of lower level (8192-16384-3=Loss:8195) by factor 6! And in the most likely case you lose three level-1 runes (Loss:3).
So even when you're not attempting to flip runes, you should, statistically, craft three level-1 runes with the highest rune you've got. Even if that means flipping them the wrong way.