how do you know when????

Hi, I was wondering how you guys know when to start the flush before harvest to wash out the nutes. I want to flush for about 1-2 weeks but how do i know when im 1-2 weeks from being done? Should i Flush when i see some orange hairs? Or...?

Thanks

i harvest when hairs are about 75% brown..
so iguess somewhere 50 to 60%