Ok, did not realize that you had a Total Control system. That's a different animal completely. As far as the level needed to keep away black algae it's not so much the FC as the ORP reading. 700 mv is on the low side of the usual range. In your climate I would think an orp reading closer to 800 mv would be better, particularly when trying to kill black algae (which is really a blue green algae or cyanobacteria) .
In marine aquairums ORP readings can accurately predict the type of algae that will grow. When the ORP is low you will get red and blue green algae (usually in that order). As ORP continues to rise (usually from the use of ozone or H2O2) then brown and finally green algae appear. This means that black algae is more likely to grow in less oxidative envionments than green algae. This is in line with what we know of black algae, that it grows under conditions of low chlorine (low oxidation potential) over an extended period of time. Another characteristic of black algae is that it has a heavy, gelatinous cell wall and it tends to grow in layers, one upon the other. Therefore, a first line of defense in killing is it frequent brushing with a wire algae bruse and using a combo wire/nylon brush for normal pool brushing (assumiong a plater pool---however black algae is not as extensive a problem in fiberglass and vinyl pools).