I am trying to do load balance over multiple ISPs, and just want to make sure the following will work.
If I go to IP/Routes and add 3 routes to ISP Gateway address A and 1 route to ISP Gateway address B. Then 80% of the traffic will go out ISP A and 20% will go out ISP B?
http://wiki.mikrotik.com/wiki/Manual:PCC
For an 80/20 split you would follow the example on that wiki page and have a total of five PCC rules, 4 of which set the routing mark for the first gateway and the 5th of which sets the routing mark for the second mark.
Unless you have all public IPs BEHIND the router PCC will work a heck of a lot better than ECMP (which is what just adding multiple routes achieves).