If the only reason you are running the higher FC level is to get the 700 mV reading from your sensor, then that is absolutely not necessary and I don't know where you get that 700 mV for one particular manufacturer's sensor is an "acceptable sanitation level". I was thinking you were saying that the sensor became unstable at higher CYA, getting more wild or varying in its readings. If that is not the case, then you should choose an FC/CYA target and then set the ORP to whatever mV reading achieves that, even if its 650 mV or even lower. As I have tried to explain in numerous posts on ORP, the absolute mV ORP reading for any given sensor is completely meaningless since there is NO consistency between the sensors from different manufacturers measuring identical pool water. As described in
this post, different sensors measuring the same water varied by more than 100 mV in 23% of the pools.
Even the manufacturer's own tables of FC (with no CYA) vs. mV ORP not only vary in absolute magnitude (i.e. a bias difference), but also in slope (change in mV for every doubling of FC). Look at
this post for example and see in
this link where your Sensorex sensor presumably shows an 83 mV increase for every doubling in FC which is absolutely ridiculous. At a pH of 7.5, they claim that 700 mV is at an FC of around 1.1 ppm with no CYA. That is certainly not what you are seeing since at 4-5 ppm FC with 20-25 ppm CYA (assuming these values are accurate) this has the same active chlorine (hypochorous acid) that is what ORP is indirectly measuring of only 0.15 ppm to 0.27 ppm FC with no CYA, though this is more in line with what other sensors report. For example, at a pH of 7.5, temp of 80ºF, 4 ppm FC with 25 ppm CYA would be 707 mV on Chemtrol, 670 mV on Oakton, 609 mV on Aquarius, and only 474 mV on Sensorex if one were to believe their table which is obviously wrong (which begs the question of what in the heck are they publishing?).
So from what you describe it sounds like you may need to run a higher FC/CYA ratio to prevent algae from growing, but you could also try to see what 4 ppm FC at 40 ppm CYA does just in case it's error in measurement of CYA that is the main problem. If that prevents the algae from growing, then you will use a lot less chlorine. Of course doing such experiments isn't easy since you'd have to dilute the water to lower the CYA level if you wanted to go back to where you were or you could run at 8 ppm FC with 40 ppm CYA or something like that and still use less chlorine than you are currently.