not easy one to answer this. To use a light bat you need decent hand eye co-ordination and fast hands thru the ball definatley helps.
too heavy a bat and you limit some shots, as you go up and play a better level(presuming you do or have done) bowlers soon work you out and if you cant cut or pull you will be peppered-limiting you scoring shots.
only thing I would say is good advise I got was visiting Andrew Kember a long time ago he helped me pick out a couple of Salix bats I came down to buy
first thing he asked me what weight I wanted, I think I said 2lb 9oz or 2lb 10 only...there were scales in the area we were in
He then said do not use the scales to pick your bat, choose the ones that feel right and narrow it down to 2 without using the scales.
I did that and then the bats were weighed and were heavier than I initially wanted.
I don't personally have a method myself but I thought this was good advice and it sunk in.