340 build

Unfortunately my yahoo mail is down, I cant receive or send any messages including private conversation messages so Ill just post a question here for A/J and look like a dumbass to everyone.

BTW I get a temporary error 2 on my yahoo mail since last night if anyone can wave a magic wand and fix that it would be appreciated

Im trying to put the steps he gave into an order I can take over to the engine and perform, Ive got this so far :

1- Put the crank in with just the front and back main-bearings, snug up the bolts to like 80% torque, not critical. Grab a sharpie a Sharpie.
2- Get any piston and rod assembly and place into #1 hole with a new bearing installed. Remember to protect the crank by placing a short piece of hose over the rod bolts.
3- Set the piston just below the deck if the piston typically sits above the deck when fully extended.
4- Lay a STRAIGHT squre piece of metal across the hole ( across the top pft he piston ) sitting on top of the deck on it's centerline in the fore/aft direction, on the wrist pin axis.
5- Set your dial indicator up on top of the deck with the pointer resting on top of the center of the keystock.
6- Now roll the crank over to push the piston up out of the hole. The piston will raise the bar and the indicator will read the positive deck-height.

What I dont get at this point is why do I need to lay the metal across the bore or across the top of the piston and measure how far up off the deck the metal is pushed, why cant I just zero out the piston at the top of the deck by fastening down a piece of straight metal across the top of the deck and across the top of the piston like I have done in previous pictures and then remove the metal and then set my dial indicator on the deck with the plunger sitting in the center of the piston and then measure how far further up the piston comes out of the hole?