If I have measured an impulse response, how can I find the level (for example in dBA)?

The input chain must be calibrated as discussed above. Then to be able to get the correct level for an impulse response we must select transient signal scope mode.

In Plot->Advanced Plot Settings..., make sure is checked to set scope mode as active.

Go to Plot->General Frequency Domain Settings... and set the scope mode to Transient signal as shown below.

More:

How do I find the background noise level of my impulse response measurement?