How can I use a magnetic declination chart to calculate true track, magnetic track and magnetic variation?
The chart shows you lines of equal declination $D$. You identify your location on the chart (which you have done and marked with an X in your second image). Then you determine on which line you are. It looks like the point is between the -26° line and the -28° line, so let's call it $D=-27^\circ$. That gives you a magnetic track of:

$$ \varphi_\text{mag} = \varphi_\text{true} - D = 70^\circ - (-27^\circ) = 97^\circ $$