Pack and Hold Time for Hot runner molds

How do you decide your holding times for hot runner molds?

I set those times based on the appearance of the molded part (sink) & also based on the dimension of the part.

I still do a gate freeze study, but realize that there is no true gate freeze and look for the weight to start leveling off. Then at that time plus about a half a second extra, I set the press.

I am still not very clear on how to 'scientifically' determine this. I do have some ideas that I am working on. I will let you know these soon. In the meanwhile, please post your comments.
Thanks,
Suhas