Let's see. Lethal range is a Fleaism. But it builds on the basic idea of convergence. If you set a convergence of 160m, then the bullets of all guns will impact at a single point in space at 160m (ignoring dispersion, shake, vibration, differences in powder load, etc). So, moving from own plane to target to infinity, the rounds angle in toward the convergence point, meet, then angle out to infinity.
Please refer to the diagram on the Sight Data page and the Spitfire data in the checklists.
The Fleaism of "lethal range" deals with these cones/angles. Assuming that a fighter's cockpit (or engine nacelle) is .8m wide, there will be a point both before and beyond the target where the cone of fire is narrower than the width of the cockpit, but not yet that single point in space. So if you set convergence at 160m, your bullets will be .8m apart at 137m, single point at 160m, and .8m apart at 183m. What does this mean?
If you are dead astern of your target, with a convergence of 160m, and aim dead center, you should have hits on target at a range of 137m - 183m. If your target is closer than 137m, or further away than 183m, then your bullets will pass harmlessly to either side of him.
Now this is based on geometry and does not account for dispersion, vibration, slip stream effects, etc. But it lets me know if I set convergence at 160m, and I fire dead astern at a target 100m or 220m away, I will most likely miss. Does this help?