That's a little tight for me. We use to run pretty tight decks on our race motors. It is hard to put an exact number on something that rocks in the bore.
When we measured the deck with piston installed, we would use an indicator. Put it on one side of the piston and rock the piston in the bore and record reading. Then do the other side of the piston. If the deck isn't square with the bore you will get two different readings. Obviously you would work off of the tightest side, or square the deck with the bore.
The tighter you run it, the more cautious you have to be with the tune and running conditions.
Aluminum rods= recomended .060"
I have run .042" on a 614" cu in BBC drag motor. I have had other customer who I wouldn't get below .060', because they weren't as careful.
That was a 6.800" rod and 4.500" range on the bore. The long rod exhands alot and the pistons rock quiet a bit in the bore. If I were building a smaller engine like a 5.7" rod with a 4.000" bore, I could tighten it up.
Steel rods= recomended .040"
On my dirt motors (SBC) I have ran as tight as .023", but that was working off of the high side of the piston rock. The high side being .015" out of the bore.
Example: If you put the indicator in the middle of the piston, and rock the piston side to side, it will not show any difference.
If you put the indicator out on the edge of the piston, it will show how much the piston rocks. On a 4.000" bore with forged pistons, it will usualy rock about .010". With your indicator zeroed on the deck, and a therotical zero deck, the indicator would read +.005" to -.005". Half way between this is .000".
Variables to consider
Expansion rate of piston material, stock cast, hypereutectic, forged high silicone, or forged low silicone. Combine this with bore diameter and entended usage, and you will get about .011" variation in bore clearence. This will result in the piston rocking more on some engines and less on others.
The theory is to work off of the center of the piston and eliminate variables, but it does the opposite when you are splitting hairs.
Expansion rate of rods, long rod versus short rod, cast versus steel versus aluminum. Each material has different expansion rates and total expansion will vary with length and temp.
Aluminum rods will obviously expand more than steel.
I have a theory that I haven't proven.
I feel that aluminum rods allow the piston to get closest to the head under full power because of heat and extreme inertial loads.
It appears to me that, steel rod motors get the closest when cold.
The only evidence I have to offer. When you build on the ragged edge, you will transfer the mill marks from the head to the piston. Not enough contact to harm it, it just keeps the carbon dusted off the piston and head.
The contact area can be anywhere from 1/4" to 1" wide without hurting it. When a customer would bring a motor to be freshened up, the steel rod motors would have a brighter contact area on the pistons. At first, I would get nervous, and for good reason. Then I noticed the difference in carbon deposits. The plugs would have evidence of running it cold and rich. The aluminum rod motors would have these deposits on the contact area, while the steel rod motors wouldn't
Theory: Steel rods expand less so the static deck clearence is tighter while cold. Combine this with a cold piston rocking in the bore and the car owner getting a little agressive with the throttle and you get contact.
Theory: Aluminum rod motors have more clearence while cold which gives more safety margin on a cold engine. Under full power, the piston and rod grow to potential contact.
This could be wrong. It was made visible to me by guys who had improperly tuned carbs. And some extreme operating conditions.
In the end, it is best to start from a known safe deck clearence. Then, every time you freshen up the engine, tighten up the deck a little and look for signs of contact. When you start dusting the carbon off the pistons and don't hurt the bearing, you have found it. 8) Once you have found it, every time you hone the cylinders, it will contact a little more. So at some point, you will have to shave the pistons a little.
That's the problem with the ragged edge, it keeps moving.
After close examination and no contact, I had a new number. 8) Along with this number, I had a set of conditions and procedures. The conditions and procedures are a very important part of the puzzle. I monitered the pistons along with cylinder wall clearence and piston rock. At about .009 to .011 cylinder wall clearence on Manley flat tops, it started showing a little contact. I don't remember how much it rocked, but it was a bunch. This is the reason the rings wouldn't last long, the head of the piston was touching above the ring. 
Hi Stubby,just proves that hands on can at times,trump the books.
