Boffin1:
So what I was trying to find out ( as I havn't actually looked into multiplexing before ) is that if I have all 800 LEDs on at once, and I want them the same brightness as if they were all on at 18mA, would I need 8 rows and 100 columns with 100 LEDs on at once ?
Yes, and you would need to run each LED at about 144mA when it is on. So that's up to 14.4A per column. Possible using mosfets and good grounding.