diff --git a/src/NTwain/Data/TWAINH.cs b/src/NTwain/Data/TWAINH.cs
index aec2969..2c79128 100644
--- a/src/NTwain/Data/TWAINH.cs
+++ b/src/NTwain/Data/TWAINH.cs
@@ -2507,7 +2507,7 @@ namespace NTwain.Data
   /// Provides the application information about the Source’s requirements and preferences regarding allocation of transfer buffer(s).
   /// </summary>
   [StructLayout(LayoutKind.Sequential, Pack = 2)]
-  public struct TW_SETUPMEMXFER
+  public partial struct TW_SETUPMEMXFER
   {
     public uint MinBufSize;
     public uint MaxBufSize;
diff --git a/src/NTwain/Data/TWAINH_EXTRAS.cs b/src/NTwain/Data/TWAINH_EXTRAS.cs
index 715b7ea..db5d492 100644
--- a/src/NTwain/Data/TWAINH_EXTRAS.cs
+++ b/src/NTwain/Data/TWAINH_EXTRAS.cs
@@ -762,6 +762,23 @@ namespace NTwain.Data
     }
   }
 
+  partial struct TW_SETUPMEMXFER
+  {
+    /// <summary>
+    /// Determines the best buffer size from values
+    /// specified by source
+    /// </summary>
+    /// <returns></returns>
+    public uint DetermineBufferSize()
+    {
+      if (Preferred != TwainConst.TWON_DONTCARE32) return Preferred;
+      if (MaxBufSize != TwainConst.TWON_DONTCARE32) return MaxBufSize;
+      if (MinBufSize != TwainConst.TWON_DONTCARE32) return MinBufSize;
+      // default to 16 kb if source doesn't really want to say what it needs
+      return 1024 * 16;
+    }
+  }
+
   //partial struct TW_DEVICEEVENT
   //{
   //    public TWDE Event { get { return (TWDE)_event; } }
diff --git a/src/NTwain/TwainAppSession.Xfers.cs b/src/NTwain/TwainAppSession.Xfers.cs
index 3909d95..f2cfb33 100644
--- a/src/NTwain/TwainAppSession.Xfers.cs
+++ b/src/NTwain/TwainAppSession.Xfers.cs
@@ -275,7 +275,7 @@ namespace NTwain
         rc = DGControl.SetupMemXfer.Get(ref _appIdentity, ref _currentDS, out TW_SETUPMEMXFER memSetup);
         if (rc == TWRC.SUCCESS)
         {
-          uint buffSize = DetermineBufferSize(memSetup);
+          uint buffSize = memSetup.DetermineBufferSize();
           var memPtr = Alloc(buffSize);
 
           TW_IMAGEMEMXFER memXfer = new()
@@ -357,20 +357,6 @@ namespace NTwain
       return WrapInSTS(rc);
     }
 
-    private static uint DetermineBufferSize(TW_SETUPMEMXFER memSetup)
-    {
-      // default to 16 kb if source doesn't really want to say what it needs
-      var buffSize = memSetup.Preferred;
-      if (buffSize != TwainConst.TWON_DONTCARE32) return buffSize;
-
-      buffSize = memSetup.MaxBufSize;
-      if (buffSize != TwainConst.TWON_DONTCARE32) return buffSize;
-
-      buffSize = memSetup.MinBufSize;
-      if (buffSize != TwainConst.TWON_DONTCARE32) return buffSize;
-
-      return 1024 * 16;
-    }
 
     private STS TransferFileImage(ref TW_PENDINGXFERS pending)
     {