path to a generated file located in the App folder on different comp.

I did not find a solution to the following problem, can anyone explain to me …

I am generating a text file through the FileMaker programm and since I do not specify it’s definite path in FileMaker, it is written into the Applications folder: /Applications/File Maker Pro/mytextfile.txt
This is done on different Apple Macs and the path may be a little different on some machines.

FileMaker is capable of using AppleScript as an extension of its own functions. I am now searching for a AppleScript function that lets me choose “mytextfile.txt” not manually with the choose function but within an AppleScript.

currently I have

set infile to (choose file)'s POSIX path
set sh to "sed 's/	/ x  /g' " & infile's quoted form & " > ~/Desktop/result.txt"
do shell script sh

That works fine for me, but how can I avoid to have to (choose file) mytextfile.txt manually ?

I tried
set infile to (path to me) as string & “mytextfile.txt”
but that gives me an error, since infile then contains the path to the AppleScript containing Application and I would need only the path to its parent folder & “mytextfile.txt”

any help would be greatly appreciated

AppleScript: 2.1.2
Browser: Firefox 3.6.21
Operating System: Mac OS X (10.4)