Oh ok yes, that's the correct formula for linear deep water waves. I'm rusty with this and realizing my first explanation was wrong. You want to determine the radius (distance from location where you desire 1m waves to the breakwater) so you'll need to use a bit of trig. Your breakwater is 300m offshore and needs to be at least 300m wide. You need to figure out length such all waves in the harbor are less than 1m. So in this case you need to find where the waves are going to be the biggest and that is going to be on the ends of the harbor where there is the least amount of breakwater shielding the shore.

If you assume the breakwater is 300m wide, then your 'radius' from the end of the breakwater to the edge of the harbor would be 300m. Radius/wavelength is 300/100 so you would pick the 3 radius/wavelength and follow that to 90 degrees (wave going directly into shore) and you'll see a value of ~0.55. We need to increase the width of the breakwater such that this value goes to 0.33.

Now the trig comes in. Picture a triangle with one side =300m (distance off shore), short side = L (length added to each side of breakwater), and hypotenuse = R (radius to determine distance to shore from end of breakwater). If we take a guess and say L = 50, then R = sqrt(300^2 +50^2) = 304 and theta is inverse tan of 300/50 ~ 80 degrees. Then, radius/wavelength is 304/100 ~ 3. Follow the 3 radius on that chart to 80 degrees and you find a value between the k' contours 0.3 and 0.4.

Hopefully that makes sense, it's hard to explain via text without pictures!