I think you have part of that backwards.
I have read that after 20 days it will loss about 4 bhn for a hardness of 7-8 is this correct or is it harder when just cast?
Usually the longer it sets the harder it gets. I've read water quenched takes a longer time to harden, but hardens more so then air cooled.
Not sure on the BHN. Tin is expensive and I'd almost start with a 1:20 mix or more.
You never mentioned what you are casting for. It can make a difference.