You are correct on the approximate 90% yield at half milk line...... 

At some point, if a machine is available, and weather works out, it's go time..... it's all too easy to get caught up in trying to get that last little bit of yield, but field loss start increasing and so does the chance that weather will wreak havoc on the best laid plans.

HERE, custom guys want to finish corn silage for all customers before doing any snaplage.  Many years, customers end up with a product that is drier than ideal   It's a challenge whether a farmer has his own machine or hires it done.  

We don't specifically wait for black layer but rely more on an accurate kernel moisture test.  

The last time we hired it harvested, it was with a newer Deere, and he had a heck of a time with wet fines filling in an area by the processing rolls..... and plugging the machine so he kept complaining that we were harvesting too wet.   I don't know if that's a universal problem on those machines or if it was operator error.  It was no fun for him laying under it and picking away at wet packed corn