Get SPL measurement of decay time (single slice of waterfall)

I'd like to get able to get a measurement of what the sound is at 150ms. Is that possible? I see it in the waterfall graph as a slice but is it possible to isolate that in the SPL graph?

Reason I ask, is that I'm attempting to do Active Room Treatment with a miniDSP. It's working to an extent but it's so much brute force trial and error. Just measurement after measurement to visually see the impact after the measurement. It's not sustainable. Below is a graph that I cobbled together. I was able to get decay modes and address those by attempting to limit what frequency comes out of the "helper" speaker with a sufficient delay (100ms plus), the create a reversing "pulse", the nailing the best times and delays down.

I got a mixer so I could do many pulses with one sub. This was before I had the mixer.

John Mulcahy said:

Use the Spectral decay graph.

Click to expand...

that gets me close to what I'm after, what I'm hoping is to be able to put a measurement in the alignment tool. The spectral decay gives me the SPL, doesn't look like the phase info is available.

I'm messing with the window settings but can't quite figure that out. Would the window settings allow me to isolate a point in time?