I wouldn't put a flat faced lifter on the cam for the RR test.

If you squeeze the oil out of one of the hyd lifters, then adjust the rocker so the plunger in the lifter is "bottomed", that will give you an effectively "solid" lifter.

You're saying the only thing under the spring is a thin shim?

Nothing to locate/center the spring like a spring cup or inner spring seat?

If that's how they are, that's a situation I would address if it were me.

When you have a spring off...... Take a pic of what's under the spring.

The .015 shims often don't hold up well to a spring that's experiencing some unhappiness.