In the layout XML file,
<BassKeyboard>
<BassRow1>
<BassButton chord="true" closing="1" closing_ext="maj" opening="8" opening_ext="maj"/>
<BassButton chord="false" closing="1" closing_ext="maj" opening="8" opening_ext="maj"/>
Seemingly, 0 is 'off', 1 is C, 2 is C# etc, 8 is G. They're semitone numbers from 1 upwards starting at some octave of C. 25 is C4, so, presumably 1 is C2. And thus presumably this is correct:
Disclaimer: I do not own an FR18 and have never played one so I have no idea what does/doesn't work.
# Note Octave
0 Off n/a
1 C 2
2 C#/Db 2
3 D 2
4 D#/Eb 2
5 E 2
6 F 2
7 F#/Gb 2
8 G 2
9 G#/Ab 2
10 A 2
11 A#/Bb 2
12 B 2
13 C 3
14 C#/Db 3
15 D 3
16 D#/Eb 3
17 E 3
18 F 3
19 F#/Gb 3
20 G 3
21 G#/Ab 3
22 A 3
23 A#/Bb 3
24 B 3
25 C 4
26 C#/Db 4
27 D 4
28 D#/Eb 4
29 E 4
30 F 4
31 F#/Gb 4
32 G 4
33 G#/Ab 4
34 A 4
35 A#/Bb 4
36 B 4
37 C 5
38 C#/Db 5
39 D 5
40 D#/Eb 5
41 E 5
42 F 5
43 F#/Gb 5
44 G 5
45 G#/Ab 5
46 A 5
47 A#/Bb 5
48 B 5
49 C 6
50 C#/Db 6
51 D 6
52 D#/Eb 6
53 E 6
54 F 6
55 F#/Gb 6
56 G 6
57 G#/Ab 6
58 A 6
59 A#/Bb 6
60 B 6
61 C 7
62 C#/Db 7
63 D 7
64 D#/Eb 7
65 E 7
66 F 7
67 F#/Gb 7
68 G 7
69 G#/Ab 7
70 A 7
71 A#/Bb 7
72 B 7
Hopefully that's a) correct & b) helpful to you.