All reputation for this information go to Nubtro.
There you get all detailed information about this topic. I will only summarize the core of this topic. This topic is very important if you upgrading your weapons or your gear to check if it's an real upgrade.
Tornadoes do 20% main hand weapon damage per tick.
When dual-wielding, tornado tick frequency depends on the attacks per second (aps) of the last weapon swing before using Sprint.
A simplified overview about Sprint RLTW (Tornado) tick frequency
Ticks = total number of ticks over 3 seconds of 1 tornado
aps = Attacks Per Second value
You have to calculate your attacks per second value manually as the ingame character screen is rounding the values and can be misleading when applied to this chart.
Attacks per second formula = ((base weapon aps * weapon IAS) + (attacks per second from legendaries and enchantress)) * (gear IAS + skill IAS + dual wield IAS)
Enchantress's aura only adds 0.03 attacks per second, instead of 3% ias in the tooltip.
Example to calculate APS and Tick frequency
MH: Echoing Fury 1,2 aps + 023 attacks per second
OH: Sword 1,4 aps
12% gear ias
15% dual wield ias
MH: 1,2 + 0,23 + 0,03 = 1,46 * 1,27 = 1,8542 => 18 ticks
OH: 1,4 + 0,23 + 0,03 = 1,66 * 1,27 = 2,1082 => 20 ticks
MH 1,46 * 1,52 = 2,2192 => 20 ticks
OH 1,66 * 1,52 = 2,5232 => 26 ticks
MH with Berserker - 2,2192 aps is very close to the next breakpoint 23 ticks with 2,22223 aps.
how much more ias on gear do we need to increase our ticks with berserker?
2,22223 / 1,46 = 1,522 => 1,522 - 1,52 = 0,02
=>1% more ias on gear would be enough
Now we have only calculated our tornado tick frequency, but if you want to compare different weapons or switching weapons, we must calculate the weapon damage.
Therefore and for the tick frequency calculation you can use this online calculator http://www.d3rawr.com/