Astrological prediction Accuracy

1.For any given astrological prediction, to make a descent assessment say with 80-90% accuracy , how many charts to be taken into consideration?

2 . May be this is a dumb question. It may make sense that – the current planet position may impact current situation on the living organisms on the earth. But, how a previous birth time planet position can influence the present time impact on this planet? Sorry if I appear too primitive, my intent is to understand and learn – but not to challenge

I understand Karma theory and need to experience the balance karmas in this birth – but unable to relate it to planet influence from past to presence .

Thanks