ok.... first off VU meters are not the same as peak meters... VU meters will 'generally' read several db lower than peak meters on high-transient material.
Easiest way is to calibrate is to send a steady signal thru the 388 that reads 0db on the VU meters (with a steady signal, peak or VU meters read pretty much the same)...
If your gain structure is matched - ie, +4dbu output to +4dbu input, or -10dbV output to -10dbV input , this will likely correspond to something very roughly like -15dBFS on your software's meters (depending on where the manufacturer calibrated their metering point).
If you have a gain mismatch of say a -10 output to a +4 input, your meter will read about 12db LOWER because the -10 output signal you'd be sending is about 12 lower than what a +4 input expects. In this case, if you have the option of a -10 input on your interface, you should use that instead - OR - you can simply use the preamp gain to increase the signal level at the expense of some noise due to the gain structure mismatch.
The idea is for the your signal level to be even all the way thru your signal chain so that you aren't boosting level unnecessarily to compensate for input/output level mismatches. The trick is discovering what "normal meter readings" actually are given your signal chain! What I described above *should* get you in the ballpark though - enough for you to get familiar with your gear interfacing.