Are the backings quartersawn, rift sawn or flat sawn?
And by "the angle increases to up to 20, 25 degrees", do you mean the GROWTH RINGS running at that angle to the side of the board?
I'm just not quite sure what you mean. It's multi-interpretative, so to speak. A picture would help a lot...
Ash is in general quite sensitive to grain run out. With its really open early wood, it tends to quickly give way between the rings. A quartersawn or rift sawn backing will tolerate a bit more ring run-outs than flat sawn.