Multiplexing LEDs - calculating resistor value

Hi all,

My previous post about speeding up multiplexing resulted in a display
with
8x48 LEDs. As I suggested and read, the LEDs are becomming less bright
when more than 2 LEDs are on in a matrx of 8x48 >=.

Can anyone help me to calculate the resistor value I need to get bright
LEDs.
Or, how can I caclulate this? Now I'm using a 100R per column which is
probably
to much.

Here is a small part of the circuit to show the current limiting
resistor of 100R.
http://imagebin.org/5461

Thanks
Greetings

.

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