power supply fine tuning

[blanik]

i have an old Roland analog synth with a built-in power supply, it converts wall voltage to +15V / Ground / -15V, i have a couple of millivolts difference between both voltages on my DMM (+15.01 and -14.98 from memory), i don't think it makes the synth out of tune (i think those values are within the tolerances) but i have another synth of the same family and it's sharp on 15V

Yamaha also made a similar synth with a similar power supply board but they designed it with two trimpots to fine tune the voltage which i think it's a great idea so i was thinking about putting trimpots on mine for the same purpose but i'm not sure where to put them, the resistors in the schem aren't where i would have thought them to be, any clues to where i should put the trimpots?

here's the schem: (it's the same board for all voltages, only the PS changes)