Straight Edge

[B2]

The tailstock ways are what i've used as the reference to this point, but I won't know if that is a reasonable assumption without putting a straightedge on them.

Do the experiment I mentioned with the gauges on the tailstock while measuring the saddle V surfaces. You will get a better feel for how things are off. Also, if you can see the surface roughness due to the original scraping (flakes) along the bed length you will then have an idea how much has worn off. If you know this then you will know the maximum that the tailstock Vs may have changed. As the scraping wears off the roughness decreases. You may need a sharper tip on your gauge to fit into the scraping holes vs peaks.

This is a challenge project, but kind of interesting metrology problem.

 

[B2]

Like you, I had to use the saddle. The bubble is dead center with the carriage against the headstock. Drops off the side when I move out about 6", then gradually comes back to dead center after 18" or so. The last third of bed travel, the bubble stays centered.

I wonder if you could do the same experiment with the level mounted on the tail stock?

 

[Shotgun]

Do the experiment I mentioned with the gauges on the tailstock while measuring the saddle V surfaces. You will get a better feel for how things are off. Also, if you can see the surface roughness due to the original scraping (flakes) along the bed length you will then have an idea how much has worn off. If you know this then you will know the maximum that the tailstock Vs may have changed. As the scraping wears off the roughness decreases. You may need a sharper tip on your gauge to fit into the scraping holes vs peaks.

This is a challenge project, but kind of interesting metrology problem.

As stated, the experiment with the tailstock is how I got the .005 figure that I used in the OP.
But, I hadn't considered trying to measure the depth of the flakes. In places, the tailstock flakes are almost gone, but not completely. Near the headstock, they're untouched as far as I can tell. That should be an easy way to see how much wear is on the tailstock Vs.