diff --git a/hooks/seatalk/0x99.js b/hooks/seatalk/0x99.js
index 10bc7e5..080fc33 100644
--- a/hooks/seatalk/0x99.js
+++ b/hooks/seatalk/0x99.js
@@ -28,31 +28,31 @@ const utils = require('@signalk/nmea0183-utilities')
                    Corresponding NMEA sentences: RMC, HDG
 */
 
+
 module.exports = function (input) {
   const { id, sentence, parts, tags } = input
 
   var XX = parseInt(parts[2], 16)
-  var value = 128 - (XX & 0x7f)
-  var s = -1
-  if (XX & (0x80 != 0)) {
-    s = 1
-  }
-  var magneticVariation = s * value
-
-  var pathValues = []
-
-  pathValues.push({
+  
+  // Convert 8-bit two's complement to signed integer
+  var magneticVariation = XX > 127 ? XX - 256 : XX
+  
+  // Documentation says: 
+  // Positive = West (negative in navigation convention)
+  // Negative = East (positive in navigation convention)
+  // So we need to invert the sign
+  magneticVariation = -magneticVariation
+  
+  var pathValues = [{
     path: 'navigation.magneticVariation',
     value: utils.transform(utils.float(magneticVariation), 'deg', 'rad'),
-  })
+  }]
 
   return {
-    updates: [
-      {
-        source: tags.source,
-        timestamp: tags.timestamp,
-        values: pathValues,
-      },
-    ],
+    updates: [{
+      source: tags.source,
+      timestamp: tags.timestamp,
+      values: pathValues,
+    }]
   }
 }