I think chopping at 50% milk line would be good for a poured concrete upright silo, but I think it would be too wet for a Harvestore.  Yeah, there are guidelines like that which you can use. However, the best way is to check whole plant moisture. Do you have a forage moisture tester, like a Koster Tester?  If not, it would be good to have one, or have access to one.  Then chop a couple of stalks and monitor whole plant moisture until it is where you want it to be. And I do believe it needs to be down to around 55% for a Harvestore.