diff --git a/src/evdev-mt-touchpad.c b/src/evdev-mt-touchpad.c
index 9b065221..88088503 100644
--- a/src/evdev-mt-touchpad.c
+++ b/src/evdev-mt-touchpad.c
@@ -994,9 +994,8 @@ tp_init_accel(struct tp_dispatch *tp, double diagonal)
 		   fixed in the actual filter code.
 		 */
 		{
-			const double MAGIC = 0.4;
-			tp->accel.x_scale_coeff *= MAGIC;
-			tp->accel.y_scale_coeff *= MAGIC;
+			tp->accel.x_scale_coeff *= TP_MAGIC_SLOWDOWN;
+			tp->accel.y_scale_coeff *= TP_MAGIC_SLOWDOWN;
 		}
 	} else {
 	/*
diff --git a/src/evdev-mt-touchpad.h b/src/evdev-mt-touchpad.h
index 1b8b560f..de8b60d7 100644
--- a/src/evdev-mt-touchpad.h
+++ b/src/evdev-mt-touchpad.h
@@ -34,6 +34,11 @@
 
 #define VENDOR_ID_APPLE 0x5ac
 
+/* Touchpad slowdown factor, see the FIXME in tp_init_accel() */
+#define TP_MAGIC_SLOWDOWN 0.4
+/* Convert mm to a distance normalized to DEFAULT_MOUSE_DPI */
+#define TP_MM_TO_DPI_NORMALIZED(mm) (DEFAULT_MOUSE_DPI/25.4 * TP_MAGIC_SLOWDOWN  * mm)
+
 enum touchpad_event {
 	TOUCHPAD_EVENT_NONE		= 0,
 	TOUCHPAD_EVENT_MOTION		= (1 << 0),
diff --git a/src/evdev.h b/src/evdev.h
index 72082e5a..9e8d6af0 100644
--- a/src/evdev.h
+++ b/src/evdev.h
@@ -33,6 +33,9 @@
 #include "libinput-private.h"
 #include "timer.h"
 
+/* The HW DPI rate we normalize to before calculating pointer acceleration */
+#define DEFAULT_MOUSE_DPI 1000
+
 enum evdev_event_type {
 	EVDEV_NONE,
 	EVDEV_ABSOLUTE_TOUCH_DOWN,
diff --git a/src/filter.h b/src/filter.h
index 9f6223d2..9e903303 100644
--- a/src/filter.h
+++ b/src/filter.h
@@ -28,9 +28,6 @@
 #include <stdbool.h>
 #include <stdint.h>
 
-/* The HW DPI rate we normalize to before calculating pointer acceleration */
-#define DEFAULT_MOUSE_DPI 1000
-
 struct motion_params {
 	double dx, dy; /* in units/ms @ DEFAULT_MOUSE_DPI resolution */
 };