I am trying to find the Thermal Resistance of the heatsink at forced convection of 300FPM.

The graphic is below. I don't know which curve to follow to find the value. I expect is has something to do with the two rectangular coordinate systems that are displayed, but I don't understand.

 

If you look at the two right angle arrows on each curve it tells which curve refers to which data.

The thermal resistance is shown by the bottom sloping curve with the right and top values, and the temperature rise above ambient is shown by the top sloping curve with the left and bottom values.

Thus at 300 FPM the thermal resistance is about 5.5°C/W.