add pcb_mount_height to common include (was tetra_pcb_mount_height)

SpHEAR_common.scad

@@ -205,6 +205,9 @@ pcb_tfit = 0.15;
 pcb_low_exp = 12.0;
 pcb_hi_exp = 8.0;
 
+// mount height
+pcb_mount_height = pcb_length - pcb_low_exp - pcb_hi_exp;
+
 // lip fit for flares
 lip_fit = 0.15;