How do you accurately measure the deck height in a bare B or RB?

The piston's relation to the deck of the block or the physical deck height of the block?

physical deck height

deck to the bearing saddle then add 1/2 of the bearing saddle bore.

I normally just hook the top of a main bore with a caliper and measure to the deck. Then add half of the main bore dimension and that gets you close.

I believe BHJ make tooling to measure it also

You can get close with a large (12") caliper to the main bearing saddle. It will take measurement skill or you could be way off.

But to get the last few thousandths, you need to put a bar in the main bores/bearings and a dial indicator with a 90 degree fixture from the deck down to the bar.

Are you trying to get within 0.010" or within 0.001" ?

One of the hardest physical measurements to make on a block at home, if not impossible to be accurate.
Heck some machine shops cant get that correct either.
Like the others said, you measure off the center of the main bore. You couldn't imagine how easy it would be to be off .008 using calipers and adding 1/2 the bearing I.D.
It be more easy to throw a old crank rod and one piston in the 4 corners and you could have a close measurement.
Then who's to say the crank index or rod length is spot on? Cause there Not.

Call your local performance machine shop. They should be able to do the measurement.

I agree with those that say a dial caliper isn't accurate enough. Even in a best case scenario with a lot of care and skill the best you can hope to determine is deck height at the mid line of the bore. It would still be all but impossible to determine if the deck was tilted ie. not exactly 90* to the crank centerline. Also the main bearing bores should be align bored prior to making any measurements. You can't do that at home so you may as well let the machine shop check the decks while it's there.

I like to set my block down on a good smooth concrete floor and a painted floor will get ye better results. make sure there ain't on gasket material on the bottom of the block where ye oil pan was. Now get ye one of these wooden yard sticks, stick it down through the top of the cylinder, let the stick catch the top of the cylinder and get your buddy to eye ball about where the center of the main crank jurnal should be cut a good notch in the stick where it catches the top of the cylinder. Now your about there take a good tape measure and measure from the bottom of the stick to the cut notch and uh you should get the same results on both sides. glad to help.

Thanks Rob that's how I do it. I just wanted to validate that I was doing it correctly.