This is a better way to do this.

src/mame/includes/blstroid.h

@@ -11,7 +11,7 @@ class blstroid_state : public atarigen_state
 public:
 	enum
 	{
-		TIMER_IRQ_OFF = TID_UNHALT_CPU+1,
+		TIMER_IRQ_OFF = TID_ATARIGEN_LAST,
 		TIMER_IRQ_ON
 	};
 

src/mame/machine/atarigen.h

@@ -206,7 +206,8 @@ public:
 		TID_6502_WRITE,
 		TID_SCANLINE_TIMER,
 		TID_ATARIVC_EOF,
-		TID_UNHALT_CPU
+		TID_UNHALT_CPU,
+		TID_ATARIGEN_LAST
 	};
 
 	// vector and early raster EAROM interface