Crusher with a polearm

Calculating from scratch (spear crusher or polearm fury):
Max: (DEX * 5 + STR) * ATK / 100 * 1.7 per hit
Min: (DEX * 5 * 0.72 + STR) * ATK / 100 * 1.7 per hit (0.72 is as Fiel ^^^ said)
Avg: (DEX * 5 * 0.86 + STR) * ATK / 100 * 1.7 per hit (0.86 is between 0.72 and 1)
For polearm crusher or spear fury, change DEX * 5 to DEX * 3.

Approximating from damage range, which is more convenient in most cases, do what JoeTang said (the 5/3 of min thing). Or if you only want a rough answer, what MasPan said (25% of range) even though that's not actually how it's calculated.

[color="#cc8899"]Eh, for not utilizing any formulas and simply reverse engineering from what I saw, I'd say I came at a pretty close number

Except yes, when you have the formula and want an exact answer, 5/3 min and 3/5 max are more accurate. 25% is pretty useful for on the fly calculations though.[/COLOR]