Why the Royal Navy tested the six-gun arrangement

Was the difference between six and eight guns minor compared with the difference in armour?

The point concerned tactical fire control: six guns had to be tested for ranging and effective fire, regardless of the separate armour comparison.

A six-gun battery had to provide enough shots for ranging and enough fire to deliver an effective blow. The Royal Navy investigated those requirements well before Renown and Repulse appeared. Courageous and Glorious, with four guns, and Furious, with two, illustrate the limitations of adopting too few guns and turrets for contemporary doctrine. Their later conversion to carriers is linked here to that inadequate main-battery arrangement. Armour thickness was a separate issue from whether a battery could apply the fire-control methods required.