Regarding estimating how long it'd take to get a full clear from a position, I think the best option is to consider it as such:
The premise is to look at how many "excess" you need of the bottom 3 ranks of each color (it's assumed you just will have a rank 4 to swallow up all the other rank 4s of a color). And pick which one's "the worst". Now in identifying excess, usually your rank 1s are going to be that worst - reason being is that a rank 1 with matching rank 2 and 3 only has a rank 1 as an "excess need" - the rank 2 and 3 will be filled when you get that rank 1 too!
But in the rare situations where you have no rank 1 or rank 2 of a given color, a higher "effective" rate can be assumed taking half the rate of the rank below it and a quarter of the rate of the rank 2 below it.
| Wind 1 | Wind 2 | Wind 3 | Fire 1 | Fire 2 | Fire 3 | Water 1 | Water 2 | Water 3 |
Base Chance | 14.00% | 8.00% | 5.00% | 19.00% | 10.00% | 7.00% | 14.00% | 8.00% | 5.00% |
Inverted | 7.14 | 12.50 | 20.00 | 5.26 | 10.00 | 14.29 | 7.14 | 12.50 | 20.00 |
Max Effective | 14.00% | 15.00% | 12.50% | 19.00% | 19.50% | 16.75% | 14.00% | 15.00% | 12.50% |
Inverted | 7.14 | 6.67 | 8.00 | 5.26 | 5.13 | 5.97 | 7.14 | 6.67 | 8.00 |
By multiplying one of the inverted rates by the amount you need, you get an approximate average distance, in spawns, until you're done. Bearing in mind that up to ~twice that distance is not entirely unlikely (but also some chance of slightly less distance).
An example from my last board that I'm currently waiting for the daily reset to get cheap on:
To complete this board, I would need:
Wind: 5 rank 1. This would create 5 rank 2 wind anyways, which i could merge to 2 rank 3 winds, and eat the last 2 winds. so no other wind needed.
Water: 4 rank 1. this would easily also swallow the 2 rank 2 waters.
Fire: 3 rank 3.
Wind 1: 5*7.14 = 35.7 spawns EV
Water 1: 4*7.14 = 28.6 spawns EV
Fire 3: (using the 16.75% effective rate for merging up extra rank 1s and 2s) 3*5.97 = 17.9 spawns EV
The simplest indicator is the 357 energy "expected" to clear the remaining wind. Of course the expectation to clear the whole board would be slightly higher than that because there's also a chance where I got appropriately lucky on wind, but am still missing those waters that aren't far behind. My guesstimate is around 38 spawns there's a 50% chance it's done. and around 60 spawns there's a 90% chance it's done.
A crude estimator would be to take the amount of tiles you need of a color and multiply it by 8 for wind/water; or 6 for fire - and the highest one you come up with, that's the number of spawns you'll still need.
Regardless, there is no way in hell I would go for a full-clear on this board
The ideal board to go for a full clear is one in which
- a) there's a lot of rank 4s, since you can swallow them all up at any time after you have 1 rank 4
- b) the colors and the ranks within the colors from 1-3 are relatively-evenly distributed where you don't need a high number of any one tile that you could get unlucky on. Needing a lot of rank 1s is the most common culprit but occasionally you can find boards that need a lot of rank 3s without lower frozen pieces to help use them up (i.e. what happened to leave me with 3 fire-3s unconsumed on this board)