Here is a small how to:
Deck height.
Factory measurement from center line of crank to top of deck.

Measure at the front of block on a chevy inline.
1. Measure bore of front main bearing. Divide # by 2 and record.
2. Measure from top of main bearing to top of deck. record #.
3. Add #'s from #1 and #2 the result is the deck height.

Compression height for zero deck
Measurement from centerline of wrist pin to top of block
1. find the Deck Height (DH) , record it
2. record length of rod(RL)
3. Get length of stroke # (SL) and divide by 2, record it.
4. Get recorded # (MB)from prior main bearing bore.
5. Take DH-(RL+SL+MB)= Compression height to zero deck.


Hope that is as clear as pea soup. Tom

If the crank is still in place but the pan is off, you can get very close by holding a straight-edge to the pan rail. The measurement from the top of the edge (the rail surface) to the top of the bore is also the deck height. This is only true of "skirtless" blocks, not of extended or "Y" blocks.
Alternative: if the front bearing ID (with the bearing in place) is known (stovebolt is 2.684"), you can also just use 1/2 the "official" ID (1.342"), which will be accurate to within a few thousandths.