Updating Space required after setting $INSTDIR

[MSG]

In my installer I am copying a 100MB file from a CD to the installation directory using FileCopy. The installer first does MUI_PAGE_COMPONENTS, then asks where to find the file from the CD (called arc.sar), and after that gets to the MUI_PAGE_DIRECTORY. What I want the installer to do is first get $INSTDIR on the DIRECTORY page, then check if $INSTDIR\arc.sar exists, and then substract arc.sar's size from the required free space, updating the Space Required: label on that same page, the DIRECTORY page.

Some parts of the script:

code:


  !insertmacro MUI_PAGE_COMPONENTS

  ;Custom page: asks where to find arc.sar

  Page custom GetSarLocation CheckSarLocation

  ;Find arc.sar in $INSTDIR and substract filesize

  !define MUI_PAGE_CUSTOMFUNCTION_PRE CheckExistingSar

  !insertmacro MUI_PAGE_DIRECTORY

  !insertmacro MUI_PAGE_INSTFILES



Function CheckExistingSar

  IfFileExists "$INSTDIR\arc.sar" 0 NoExistingSar

  Push "$INSTDIR\arc.sar"

  Call FileSizeNew

  Pop $EXISTING_SAR_SIZE

  IntOp $EXISTING_SAR_SIZE $EXISTING_SAR_SIZE % -1000

  SectionGetSize CommonInstall $2

  IntOp $2 $2 - EXISTING_SAR_SIZE

  SectionSetSize CommonInstall $2



  NoExistingSar:

FunctionEnd

As you can see I've tried to use PAGE_CUSTOMFUNCTION_PRE on PAGE_DIRECTORY, and indeed the function CheckExistingSar updates the size of the CommonInstall section succesfully if it can find arc.sar. Only the 'Space required' value does not change, and the CheckExistingSar function is in fact called only once. Entering a different install path does not call the function again.

I've read some hints about using FindWindow etc commands to manually influence the label, but I have no clue how to go about this. Any help would be greatly appreciated.