I'm sure you'll get other suggestions also. But I've had very good luck with this one.(Image removed from quote.)
I don't use a special site when I'm flying the P-39 and I think the same philosophy would apply in any situation where you are planning for air to air combat. Set your convergence and fire at that convergence with the tater guns which in most cases is going to be fairly close in because of their relative slow velocity and rate of fire. The one exception I could see would be in the case of GV hunting in the Hurricane IID where you may want a convergence beyond 650 yards. I haven't flown the the Hurricane much so I'm not sure off hand what range the 40mm guns are effective at.