SetOverwrite, IfErrors & File space calculation issue

[mrtech]

I've been trying to get this working for a little while now and here is what I've come up with:

Situation:
- my VB app downloads and runs nsis installer via shellexecute as an upgrade
- installer/app have a timing issue and needs the first error "Can't Overwrite" msg back in order for the app to finish the shellexecute and unload itself

Fix:
SetOverwrite try

File mrsystray.exe ; might still be in use or unloading

IfErrors 0 NoError
Sleep 100
File mrsystray.exe
NoError:

Problem:
Though the installer is still very small, the required file space is reported as doubled of what you really need.

Question:
Is there anyway of adding a /dontadd or /ignoresize to the File command or like with the AddSize add a SubstractSize

In the meantime I'm using AddSize -252 but the size of my app changes from build to build. Tried using the FileSizeNew script but it returned the amount in bytes and could really get it to compile.

Any suggestions would be grateful, thanks.

Mel