Some might find this useful, list of layers in use + 53 & 54 for connecting digital & analogue grounds together:
Set Used_Layers 1 2 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40
41 42 43 44 45 46 47 48 49 50
51 52 53 54 56
91 92 93 94 95 96
101 102 104 116
121 122
151
200 201 202 203 204 205 206 207 208
254
HTH
Leo