How do I measure blur in Photoshop

[RichardSwearinger]

I am compositing a very sharp image onto a less sharp image and the sharper image sticks out like a sore thumb.

I need to blur the sharp image so that it matches the soft one. Eyeballing or guesstimating is out of the question, non-negotiable, can’t be done.

So what tools are there in Photoshop or Lightroom that would allow me to match the blur between them?

I know that one of you geniuses must have figured out a way to do it using a high pass layer or one of the averaging functions or blend modes or some other clever, non-obvious method. 

[Trevor.Dennis]

This might help.

Ctrl click the composited object layer to load as a selection.

Hit Q to go into quick mask.

Apply Gaussian Blur to the Quick Mask to match the rest of the image. 

When it looks right, hit Q again to come out of Quick mask, and you are left with the right degree of feather.

You could add a Layer mask and shift it in or out to finish.

Or make the layer a Smart Object.

Guess the Blur and if wrong, go back in and repeat until right.

Or ctrl click to make a selection

Use select > Modify to expand or contract a few pixels.

Add layer mask, and use Feather in Mask Properties.  

Use Shift Edge in Mask Properties to fine tune mask size.

The last two suggestions are non destructive.

Ctrl click to load selection

Make the selection into a workpath (it won't be flash, but it doesn't need to).

Stroke the Workpath with the Blur tool, (I use this a lot with all sorts of tools).

[Trevor.Dennis]

You actually asked how to measure blur.  If you have an area of flat tone, then you can do just that.

I used Gaussian Blur with a value of 10 pixels, so I don't know exactly how it works, because fully opaque to fully transparent stretches 50 pixels.  

[RichardSwearinger]

Sorry, I'm not a math person, so I'll need help here. Are you saying that the levels of black ink correspond to the levels of sharpness?