diff --git a/src/ConfigFile.cpp b/src/ConfigFile.cpp
index ea4cc32..16a443e 100644
--- a/src/ConfigFile.cpp
+++ b/src/ConfigFile.cpp
@@ -574,10 +574,8 @@ bool ShadeConfigFile::restoreFile(SomfyShadeController *s, const char *filename,
   }
   else {
     ESP_LOGI(TAG, "Shade data ignored");
-    // FF past the shades and groups.
-    this->file.seek(this->file.position()
-      + (this->header.shadeRecords * this->header.shadeRecordSize)
-      + (this->header.groupRecords * this->header.groupRecordSize), SeekSet);  // Start at the beginning of the file after the header.
+    // FF past the rooms, shades and groups.
+    this->file.seek(this->file.position() + shadeModelBytes(this->header), SeekSet);
   }
   if(opts.repeaters) {
     ESP_LOGI(TAG, "Restoring Repeaters...");
diff --git a/src/ConfigFile.h b/src/ConfigFile.h
index 011e7a9..3759ba2 100644
--- a/src/ConfigFile.h
+++ b/src/ConfigFile.h
@@ -2,6 +2,7 @@
 #include <LittleFS.h>
 #include "Somfy.h"
 #include "ConfigSettings.h"
+#include "ConfigHeader.h"
 
 #ifndef configfile_h
 #define configfile_h
@@ -12,22 +13,6 @@
 #define CFG_TOK_QUOTE '"'
 
 
-struct config_header_t {
-  uint8_t version = 1;
-  uint8_t repeaterRecords = 0;
-  uint8_t repeaterRecordSize = 0;
-  uint8_t roomRecords = 0;
-  uint16_t roomRecordSize = 0;
-  uint16_t shadeRecordSize = 0;
-  uint8_t shadeRecords = 0;
-  uint16_t groupRecordSize = 0;
-  uint8_t groupRecords = 0;
-  uint16_t settingsRecordSize = 0;
-  uint16_t netRecordSize = 0;
-  uint16_t transRecordSize = 0;
-  char serverId[10] = ""; // This must match the server id size in the ConfigSettings.
-  int8_t length = 0;
-};
 class ConfigFile {
   protected:
     File file;
diff --git a/src/ConfigHeader.h b/src/ConfigHeader.h
new file mode 100644
index 0000000..bf21a34
--- /dev/null
+++ b/src/ConfigHeader.h
@@ -0,0 +1,33 @@
+#ifndef CONFIG_HEADER_H
+#define CONFIG_HEADER_H
+#include <stdint.h>
+
+// The header of a shades.cfg file: its version and how many records of each
+// kind follow, and their sizes.  Kept free of the file system so it can be
+// unit tested on the PC (pio test -e native).
+struct config_header_t {
+  uint8_t version = 1;
+  uint8_t repeaterRecords = 0;
+  uint8_t repeaterRecordSize = 0;
+  uint8_t roomRecords = 0;
+  uint16_t roomRecordSize = 0;
+  uint16_t shadeRecordSize = 0;
+  uint8_t shadeRecords = 0;
+  uint16_t groupRecordSize = 0;
+  uint8_t groupRecords = 0;
+  uint16_t settingsRecordSize = 0;
+  uint16_t netRecordSize = 0;
+  uint16_t transRecordSize = 0;
+  char serverId[10] = ""; // This must match the server id size in the ConfigSettings.
+  int8_t length = 0;
+};
+
+// The bytes of the shade model's records, which follow the header in this
+// order: rooms, shades, groups.  A restore that leaves the shades out skips
+// this many bytes.
+inline uint32_t shadeModelBytes(const config_header_t &header) {
+  return static_cast<uint32_t>(header.roomRecords) * header.roomRecordSize
+    + static_cast<uint32_t>(header.shadeRecords) * header.shadeRecordSize
+    + static_cast<uint32_t>(header.groupRecords) * header.groupRecordSize;
+}
+#endif
diff --git a/test/test_configheader/test_configheader.cpp b/test/test_configheader/test_configheader.cpp
new file mode 100644
index 0000000..1b37ef2
--- /dev/null
+++ b/test/test_configheader/test_configheader.cpp
@@ -0,0 +1,55 @@
+#include <unity.h>
+#include "ConfigHeader.h"
+
+void setUp() {}
+void tearDown() {}
+
+static config_header_t header() {
+  config_header_t h;
+  h.version = 22;
+  h.roomRecords = 3;
+  h.roomRecordSize = 29;
+  h.shadeRecords = 5;
+  h.shadeRecordSize = 276;
+  h.groupRecords = 2;
+  h.groupRecordSize = 200;
+  h.repeaterRecords = 1;
+  h.repeaterRecordSize = 77;
+  h.settingsRecordSize = 100;
+  return h;
+}
+
+static void test_shade_model_counts_rooms_shades_and_groups() {
+  // A restore that leaves the shades out has to skip the rooms too, or it
+  // reads the repeaters and settings from the room records.
+  TEST_ASSERT_EQUAL_UINT32(3 * 29 + 5 * 276 + 2 * 200, shadeModelBytes(header()));
+}
+static void test_shade_model_leaves_out_the_rest() {
+  config_header_t h = header();
+  const uint32_t before = shadeModelBytes(h);
+  h.repeaterRecordSize = 1;
+  h.settingsRecordSize = 1;
+  h.netRecordSize = 1;
+  h.transRecordSize = 1;
+  TEST_ASSERT_EQUAL_UINT32(before, shadeModelBytes(h));
+}
+static void test_an_old_file_without_rooms() {
+  config_header_t h = header();
+  h.roomRecords = 0;
+  TEST_ASSERT_EQUAL_UINT32(5 * 276 + 2 * 200, shadeModelBytes(h));
+}
+static void test_a_full_model_does_not_overflow() {
+  config_header_t h;
+  h.roomRecords = h.shadeRecords = h.groupRecords = 255;
+  h.roomRecordSize = h.shadeRecordSize = h.groupRecordSize = 0xFFFF;
+  TEST_ASSERT_EQUAL_UINT32(3UL * 255 * 0xFFFF, shadeModelBytes(h));
+}
+
+int main(int argc, char **argv) {
+  UNITY_BEGIN();
+  RUN_TEST(test_shade_model_counts_rooms_shades_and_groups);
+  RUN_TEST(test_shade_model_leaves_out_the_rest);
+  RUN_TEST(test_an_old_file_without_rooms);
+  RUN_TEST(test_a_full_model_does_not_overflow);
+  return UNITY_END();
+}