diff --git a/src/modules/fsresizer/ResizeFSJob.cpp b/src/modules/fsresizer/ResizeFSJob.cpp
index 0d10c5583..f71f010bd 100644
--- a/src/modules/fsresizer/ResizeFSJob.cpp
+++ b/src/modules/fsresizer/ResizeFSJob.cpp
@@ -68,31 +68,48 @@ ResizeFSJob::RelativeSize::RelativeSize( const QString& s)
 {
     matchUnitSuffix( s, "%", Percent, m_value, m_unit );
     matchUnitSuffix( s, "MiB", Absolute, m_value, m_unit );
+    
+    if ( ( unit() == Percent ) && ( value() > 100 ) )
+    {
+        cDebug() << "Percent value" << value() << "is not valid.";
+        m_value = 0;
+        m_unit = None;
+    }
 
     if ( !m_value )
         m_unit = None;
 }
 
 qint64
-ResizeFSJob::RelativeSize::apply( Device* d )
+ResizeFSJob::RelativeSize::apply( qint64 totalSectors , qint64 sectorSize )
 {
     if ( !isValid() )
         return -1;
-
+    if ( sectorSize < 1 )
+        return -1;
+    
     switch( m_unit )
     {
     case None:
         return -1;
     case Absolute:
-        return CalamaresUtils::MiBtoBytes( value() ) / d->logicalSize();
+        return CalamaresUtils::MiBtoBytes( value() ) / sectorSize;
     case Percent:
-        return d->logicalSize() * value() / 100;
+        if ( value() == 100 )
+            return totalSectors;  // Common-case, avoid futzing around
+        else
+            return totalSectors * value() / 100;
     }
 
     // notreached
     return -1;
 }
 
+qint64
+ResizeFSJob::RelativeSize::apply( Device* d )
+{
+    return apply( d->totalLogical(), d->logicalSize() );
+}
 
 ResizeFSJob::ResizeFSJob( QObject* parent )
     : Calamares::CppJob( parent )
@@ -150,9 +167,11 @@ qint64
 ResizeFSJob::findGrownEnd(ResizeFSJob::PartitionMatch m)
 {
     if ( !m.first || !m.second )
-        return -1;
+        return -1;  // Missing device data
     if ( !ResizeOperation::canGrow( m.second ) )
-        return -1;
+        return -1;  // Operation is doomed
+    if ( !m_size.isValid() )
+        return -1;  // Must have a grow-size
 
     cDebug() << "Containing device size" << m.first->totalLogical();
     qint64 last_available = m.first->totalLogical() - 1;  // Numbered from 0
@@ -198,6 +217,13 @@ ResizeFSJob::findGrownEnd(ResizeFSJob::PartitionMatch m)
         }
     }
 
+    qint64 wanted = m_size.apply( expand, m.first->logicalSize() );
+    if ( wanted < expand )
+    {
+        cDebug() << ".. only growing by" << wanted << "instead of full" << expand;
+        last_available -= ( expand - wanted );
+    }
+    
     return last_available;
 }
 
diff --git a/src/modules/fsresizer/ResizeFSJob.h b/src/modules/fsresizer/ResizeFSJob.h
index 36527ab56..063495e7d 100644
--- a/src/modules/fsresizer/ResizeFSJob.h
+++ b/src/modules/fsresizer/ResizeFSJob.h
@@ -64,15 +64,23 @@ public:
             return ( unit() != None ) && ( value() > 0 );
         }
 
-        /** @brief Apply this size to the given device @p d
+        /** @brief Apply this size to the number of sectors @p totalSectors .
          *
+         * Each sector has size @p sectorSize , for converting absolute
+         * sizes in MiB to sector counts.
+         * 
          * For invalid sizes, returns -1.
          * For absolute sizes, returns the number of sectors needed.
-         * For percent sizes, returns the number of sectors matching
-         *     that percentage of the device size.
+         * For percent sizes, returns that percent of the number of sectors.
+         */
+        qint64 apply( qint64 totalSectors, qint64 sectorSize );
+        
+        /** @brief Apply this size to the given device.
+         * 
+         * Equivalent to apply( d->totalLogical(), d->logicalSize() )
          */
         qint64 apply( Device* d );
-
+        
     private:
         int m_value;
         Unit m_unit;
diff --git a/src/modules/fsresizer/fsresizer.conf b/src/modules/fsresizer/fsresizer.conf
index f02983e45..06141fc7d 100644
--- a/src/modules/fsresizer/fsresizer.conf
+++ b/src/modules/fsresizer/fsresizer.conf
@@ -23,6 +23,8 @@ fs: /
 # how big the card is), use MiB as suffix in that case.
 # If missing, then it's assumed to be 0, and no resizing
 # will happen.
+#
+# Percentages apply to **available space**.
 size: 100%
 
 # Resizing might not be worth it, though. Set the minimum
@@ -30,4 +32,6 @@ size: 100%
 # resizing is skipped. Can be in percentage or absolute
 # size, as above. If missing, then it's assumed to be 0,
 # which means resizing is always worthwhile.
+#
+# Percentages apply to **total device size**.
 atleast: 1000MiB