I've run in to this on a few builds and most of the time I either used a Lin pot instead of a Log pot, or mixed up a resistor by a factor of 10 (10k when it should have been 1k or 100k). The +-15V PSU on this build already gives crazy headroom & volume compared to a normal +9V pedal, so it's worth checking the taper & values on your build.
I've also built a few of those LT1054 Bipolar Voltage Converters, and with 1N5817 diodes, they tend to kick out closer to +-17V than +-15V. If you have a multimeter, check the output of that board. If you want to lower that a bit, you can use 1N4001 diodes instead of 1N5817s. You can also try using a 1N4001 in series with the + & - rails for ~1V drop on each. Put a 20ohm series current-limiting resistor after that and you should be much closer to +-15V
If you have a multimeter, you can also use the AC Volts setting on the pedal output to see what kind of output voltage range the pot is giving you. Place the black lead on your ground & red lead on the board output. Either play guitar at a consistent volume or use a looper to read the voltage, which is your guitar signal in Volts RMS. Adjust the Master pot and take readings at 10%, 50%, & 90%. A typical guitar signal is somewhere around 70mV RMS (0.071V), but can swing above 500mV RMS when you strum or pluck the strings hard. Generally, anything over 1V RMS at any Master pot settings will clip whatever you have the pedal hooked up to, and will drive an amp to heavy overdrive.