How to calculate the wind variation? Given the:

True Heading = 070 degrees

True Track   = 061 degrees

True Air Speed = 120 knots

Ground Speed = 118 knots

what is the wind variation?

these are the steps I am taking to calculate the wind variation:

1 - put the true heading on top - the true heading is 070 degrees



2 - put the true air speed inside the little circle - the true air speed is 120 knots



3 - mark the drift ( true heading - true track) = (070 - 061) = 9 degrees on that line



4 - draw down to the ground speed (118 knots)



5 - bring the little blue circle back to zero



6 - rotate the disk so that the cross is on the central line



there I can see that the wind comes from 151 degrees at 19 knots.

Questions:

will this always be correct?

Could I have calculated it through a formula (without using the computer)?
Sure it can.  It's just vector math. 

$$\vec{A} + \vec{B} = \vec{C}$$

If $\vec{C}$ is your ground speed vector and $\vec{A}$ is your airspeed vector (magnetic heading with true airspeed as a magnitude, then $\vec{B}$, the wind vector, can be solved for by

$$\vec{B} = \vec{C}- \vec{A}$$

Electronic flight computers and E6B apps for tablet computers/smartphones can do the same thing.  But it's nice to have some basic skills with a whiz wheel.