InnoSetup - Set DefaultDirName at runtime - inno-setup

Question

I have an application that needs to allow for up to three simultaneous installations on the same machine.
For some reason, the following code behaves as if UsePreviousAppDir is set to yes. The second time I install the app, the path ends up mangled.
The value I want to see is
C:\Our App\install_x where x corresponds to the user's selection.
It works the first time, but the second run results in something like this:
C:\Our App\install_x\install_y, where x corresponds to the value selected with the first installation and y corresponds to the value selected during this installation.
The install version is a radio button selection grabbed from the first screen in the installer. How do I eliminate this issue?
Setup section:
[Setup]
AppName=Our App
AppId=Our App
AppVerName=Our App Version(CM)
DefaultDirName=C:\Our App
DefaultGroupName=Our Group Name
OutputDir=..\
OutputBaseFilename=mm_setup
DisableStartupPrompt=yes
Compression=zip
UsePreviousAppDir=no
VersionInfoDescription=Our App Setup
CreateUninstallRegKey=no
DirExistsWarning=no
And the method where I set the install version:
procedure gSetVersion;
begin
if gVersionPage.SelectedValueIndex = 0 then
begin
gInstallArea := 'install_a';
end
else if gVersionPage.SelectedValueIndex = 1 then
begin
gInstallArea := 'install_b';
end
else if gVersionPage.SelectedValueIndex = 2 then
begin
gInstallArea := 'install_c';
end
WizardForm.DirEdit.Text := WizardDirValue + '\' + gInstallArea;
end;

Answer 1

Solved the problem via the following hack. Not sure why it was necessary to manually edit the string when the `UsePreviousAppDir=no' was set, but this works
procedure gSetVersion;
var
installVersionIndex: Integer;
installDir: String;
begin
case gVersionPage.SelectedValueIndex of
0: gInstallArea := 'install_a';
1: gInstallArea := 'install_b';
2: gInstallArea := 'install_c';
end
//Set the default installation folder.
//This is necessary because InnoSetup intermittently
//ignores the 'UsePreviousAppDir=no' [Setup] directive
//and because the 'DefaultDirName' directive gets populated
//prior to the user selecting the install version
installVersionIndex := Pos('install_', WizardDirValue);
installDir := WizardDirValue;
if installVersionIndex > 0 then
begin
Delete(installDir, installVersionIndex, 20);
end
WizardForm.DirEdit.Text := installDir + '\' + gInstallArea;
end;