Flow rate of water in a gutter system
You have a lot of flexibility, as if there is not enough water flowing out the depth in the gutter will increase, increasing the pressure and flow rate.  Here is an online calculator which says a 1/8" hole at 0.036 psig will flow 3.7 gallons per hour.  I picked the pressure to be roughly one inch deep water.  The flow rate increases with the square root of the pressure.