relative path in BAT script

Here is my own program folder on my USB drive:

Program\
     run.bat
     bin\
         config.ini
         Iris.exe
         library.dll
         etc.

I would like to use run.bat to start Iris.exe

I cannot use this: F:/Program/bin/Iris.exe like a shortcut, because sometimes it does not attach as drive F: (e.g. E: or G:)

What do I need to write in the bat file to work regardless of the drive letter?

I tried this in the BAT file:

"\bin\Iris.exe"

But it does not work.