dosing

Guys,
Deciding between 2 part or 3 part dosing..If I am doing Alk and Cal does it only make sense to add the addition Mag and get a 3 part auto Doser or would the Alk/Cal keep my mag perams in check on its own? Mostly softies and soft corals at this point but will want to dabble in the lps and sps as soon as my tank has aged enough in the near future so just setting it all up now to start the process. As it is I am dosing manually with the 2 part Bionic(just started today actually)15 miles of each in morning because I seem to struggle with mostly Cal and Mag low levels but Alk is only in the 7 range also so...
I do 15% water changes weekly using the coral pro.
Readings as of yesterday-
Alk 7
Cal 410
Mag 1200
Temp 79
PH 8.40
NITRATE 0
PHOSPHATE 0
75 gallon reef
30 gallon sump/fuge
2 radions pro
2 MP40's
Apex mounted but only reading PH/Temp over the fusion because at this point the whole programming thing has me in a brain lock!