The Ash I milled 14-15 months ago here in Missouri has not cupped very much. The slabs came off the log that stood on the stump in my avatar. Most 37" wide by just over seven feet long.heres one recently that ramps were cut from the slabs that were second in order of cut or same placing on the last cuts cupped the most. Obviously your milage may vary from the difference in weather and exact species. Just my $.02What about beams (6" by 12" by 12') Same 8% rule apply? Ash and hemlock are the materials.
Stay safe and enjoy