If crit range/multiplier is:
20/x2 then multiply damage by 1.05
20/x3 or 19-20/x2 then multiply damage by 1.10
20/x4 or 18-20/x2 then multiply damage by 1.15
19-20/x3 or 17-20/x2 then multiply damage by 1.20
18-20/x3, 19-20/x4, or 15-20/x2 then multiply damage by 1.30
18-20/x4 or 12-20/x2 then multiply damage by 1.45
Great shortcut.

With that chart, the quickie formula would be:
((21 * number of attacks) - (sum of rolls needed to hit)) * average damage * crit factor / 20
For example, 2d6+12 19-20/x4, needing rolls of 5, 10, 15 to hit: (63 - 30) * 17 * 1.3 / 20 = 36.465.
And if you have energy enchantments it's just a little more complex:
((21 * number of attacks) - (sum of rolls needed to hit)) * avg normal dmg * (1 + ((crit factor-1) * avg crit dmg / avg normal dmg)) / 20
For example, give that weapon above a 1d6 flaming enchantment and you have:
(63-30) * 20.5 * (1 + (.3 * 17 / 20.5)) / 20 = 42.24.
Or flaming burst (why, God, why?):
(63-30) * 20.5 * (1 + (.3 * 22.5 / 20.5)) / 20 = 44.9625.
Or to remind everyone why burst sucks, trade it for another +1:
(63-27) * 21.5 * (1 + (.3 * 18 / 21.5)) / 20 = 48.42