I'd have to look it up to be 100% sure of my answer. I know it's not an average. I believe it looked to be the middle quality, vitality, though last thread on the matter suggested more testing was needed to verify..
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The middle quality ,vitubstance, is sadly not the one used in calculating effectiveness. I was under the same impression till i got yelled at for being a derp and tested it and saw that it is indeed the average that matters. Do note however that it is likely a linear average not some fancy schmancy quadratic formula. (should probably be tested further by people nerdier than myself)
With the numbers that were posted in the earlier thread I'm referncing, an average wasn't working in the calculation, either, but as I said, nothing came out conclusive from that thread and further testing was needed to verify any ideas.
Pickaxe or axe quality is same as qurios quality PickQ = sqrt((E*E + S*S + V*V)/3) without rounding down.
Mining power is sqrt(PickQ * Str)
"Just slightly too hard" means youll need up to 5 more power to mine this tile "Slightly too hard": 6-10 "Much too hard": 11-20 "Very much too hard": 21-40 "Impossible to scratch": >40