Looking for the safest resistance for Parallel-series LEDs
You will NOT be able to pick a value to guarantee just under 100mA. 100mA is also the MAX... life will be short at that, 50mA is likely better. 

In order to calculate R you need to subtract the total forward voltage of the diodes from the supply voltage. Normally you would use the typical value of 1.4V shown here, that's 5.6V across the diodes so 3.4V remaining. That would be an R value of 34R for 100mA.

HOWEVER, since there is no minimum value for Vf you can assume some will be less than 1.4V, so for 34R, your current will exceed the max of 100mA.

As such you need to guess and derate the current to perhaps 50%.. e.g. 50mA. Your R value will then be @75R.

But if you need it that tight to the limit you would be better driving them with constant current drivers or using a select on test resistance value.

EDIT Since you changed it to 80mA, R value would be 43R half watts.