For CBR -q 3 is probably correct. I just tested CBR @ 128 kbps. "-b 128 -q 3" and "-b 128" produced identical files.
With the default VBR mode (--vbr-new) the -q switch seems to have only two effective modes: the default mode which is used without a -q switch and with -q 0...4, and another mode which is used with -q 5...9. (I tested -V 5 with all possible -q values.)
The "other" mode seems to be quite similar to the default mode. According to EncSpot Pro it uses the same MP3 encoding features as the default mode. Only the bitrate distribution and the bit reservoir values differ slighly in my test files.
EDIT
The default -q mode:
Bitrates:
----------------------------------------------------
32 0.0%
64 0.0%
80 0.2%
96 ||| 3.8%
112 ||||||||||||||||||||||||||| 28.4%
128 |||||||||||||||||||||||||||||||||||||||| 40.6%
160 |||||||||||||| 14.6%
192 |||||| 6.2%
224 ||| 3.3%
256 || 2.2%
320 0.7%
----------------------------------------------------
Type : mpeg 1 layer III
Bitrate : 137
Mode : joint stereo
Frequency : 44100 Hz
Frames : 8839
ID3v2 Size : 2180
First Frame Pos : 2180
Length : 00:03:50
Max. Reservoir : 208
Av. Reservoir : 36
Emphasis : none
Scalefac : not used
Bad Last Frame : no
Encoder : Lame 3.98
Lame Header:
Quality : 50
Version String : Lame 3.98
Tag Revision : 0
VBR Method : vbr-mtrh
Lowpass Filter : 16500
Psycho-acoustic Model : nspsytune
Safe Joint Stereo : no
nogap (continued) : no
nogap (continuation) : no
ATH Type : 4
ABR Bitrate : 32
Noise Shaping : 1
Stereo Mode : Joint Stereo
Unwise Settings Used : no
Input Frequency : 44.1kHz
--[ EncSpot 2.1 ]--
The "other" -q mode:
Bitrates:
----------------------------------------------------
40 0.0%
64 0.0%
80 0.2%
96 ||| 3.9%
112 |||||||||||||||||||||||| 25.1%
128 |||||||||||||||||||||||||||||||||||||||| 41.4%
160 |||||||||||||||| 17.0%
192 ||||| 6.1%
224 ||| 3.4%
256 || 2.3%
320 0.6%
----------------------------------------------------
Type : mpeg 1 layer III
Bitrate : 139
Mode : joint stereo
Frequency : 44100 Hz
Frames : 8839
ID3v2 Size : 2180
First Frame Pos : 2180
Length : 00:03:50
Max. Reservoir : 208
Av. Reservoir : 37
Emphasis : none
Scalefac : not used
Bad Last Frame : no
Encoder : Lame 3.98
Lame Header:
Quality : 45
Version String : Lame 3.98
Tag Revision : 0
VBR Method : vbr-mtrh
Lowpass Filter : 16500
Psycho-acoustic Model : nspsytune
Safe Joint Stereo : no
nogap (continued) : no
nogap (continuation) : no
ATH Type : 4
ABR Bitrate : 32
Noise Shaping : 1
Stereo Mode : Joint Stereo
Unwise Settings Used : no
Input Frequency : 44.1kHz
--[ EncSpot 2.1 ]--