That is a very general, ballpark, rough, rule of thumb that attempts to average many, many, many variables.
I checked some of my stock last week with a meter. 10/4 pecan slabs I milled March '06 gave me readings of 16% +-1% which is close enough to EMC for my area for them to be used. If I followed the 1" rule of thumb I would be waiting for another year and a half. Even if I dried them in a kiln they will eventually equalize back to whatever the current weather conditions dictate.
I stole this from an undisclosed source. It may be useful to someone.
Well said aggie... example... I have had some 4/4 cherry dry to 20% in 6 weeks, and then another cherry, same thickness but took almost 6 months. I have too much else going on, and mill too many different kinds of trees at different times to keep track of enough variables to nail it down what factors effect what exactly. If this is all I did, I might eventually get somewhere with that. Botton line is, the wood is dry when the meter says it is
In general, 4/4 or 5/4 anything rarely takes more than 6-8 months to dry. Often only 3-4 months. But just too many variables.
You asked if I had any sweetgum slab pics. No, but if I did they wouldn't impress you much. I have milled sweetgum in the past... talking about this stuff...
http://www.cnr.vt.edu/dendro/dendrology/syllabus/factsheet.cfm?ID=53 and gotten beautiful reddish brown heartwood. Unfortunately, this particular tree that I milled last spring was almost pure white sapwood the whole 24 inches dia. Bummer. So got lots of hard solid homogeneous white hardwood boards out of it, but no beautiful colorful grain as I had hoped when I agreed to take the tree (before it was actually cut down). I might try and stain some of it and see what that does for me. Right now all of it, mostly 5/4, but several huge 12/4 slabs, are buried under a bunch of white oak I stickered on top of it. I'll pull the whole pile apart in the spring. By that time it should all be down to at least 20% at which time I un-sticker and store the wood away. As you can tell by your little fact sheet on MC and humidity, in the humid summer (here on east coast) it won't dry to much less than that no matter how long it sits there.
I felled a fairly large chestnut for a friend of mine late last fall but didn't get it milled before winter and other things got in the way. Plan on doing that soon, and will post pics of that when I do. For those of you who have never seen it, Chestnut is kindof a unique wood. It is often a beautiful light brown and has lots of squirrelly grain in the crotches etc. Should make for some nice milling pics.