You asked about how to compare your light meter with another camera. Analog or digital doesn't matter.

There should be an indication what combination of shutter speed and aperture to choose. Maybe an lcd-screen.

What you do is you take your other camera and see what combination that camera chooses (when it is set for ISO 400). Point the camera towards a a large evenly lit object, such as a wall for example. For this test, make sure that there are no brighter or darker areas. Ok, you got a value, let's say ISO 400, f16, speed 400. Then point your Pentax towards exactly the same object, not once but say five times. Do you get consistent values? Are the values close to what you got with your other camera? If not, well you might have a problem.