Hey Mark
The shower and ring circuits are two different things, the column for ring is for the ring continuity as you say, and this is only for when you do a ring circuit, anythnig else lights, shower, cooker etc you should put a dash or a line through to show its n/a, you need to make sure you do your r1+r2 for the shower, and this will be marked in the column for it, the rest insulation resistance, impedance tests are all straight forward enough.
if you need anymore help or get stuck with any of the testing, don't hesitate to come back and ask