It's possible I'm mistaken, but I think FM uses word processor-like tabs. It moves you to the next tab space. I have not found a way to alter the tab stops in the calculation dialog without some hackery.
1. If you want to set tab positions in a particular field instance or text object you can do that in the Appearance inspector (see screenshot). There the default position when you start from scratch seems to be 36pt, but of course you can set each tab to whatever you wish.
2. I've never thought to check the width of tabs inserted into calcs (which I assume you know you do using option/alt + tab), but in the attached screenshot you can see that on my screen a tab occupies about 5 letter spaces, which is between what you are seeing and what you'd prefer. Don't know what that proves: it's machine or OS dependent, perhaps?
I checked on Windows 7, the tab width is about 8 characters. I say about because it seem to be set in pixel width, not in character width. If character set is Tahoma, then it's very close to 8 characters in width.
And looks like there is no way in changing that. I googled that, and haven't found anything. This seems a system wide definition in Windows, or for these kinds of controls. I made small experiment in VBA, by adding a text box control to a form. I can insert Tabs by doing Ctrl+Tab, again it's about 8 characters wide and there is no parameter to set that.
A tab was type to the left of that sentence
See, in here it's shorter .
Looks like I will have to live with that .
Re: "I say about because it seem to be set in pixel width, not in character width"
This would be because most fonts use variable spacing—each letter only taks up as much horizontal space as it needs (hence the printing measures, the en and the em). You could try it with a fixed space font such as courier and see how many characters match the apparent pixel width of tabs.
As for your 7 or 8 characters vs my 5ish, I gueass that's a Windows/Mac thing.
If you are allowed to install program, AutoIt http://www.autoitscript.com/site/autoit/
can set tab width of FM dialog.
The script below wait the dialog to open, and set tab width to 10px.
(make it to .exe and put it to startup folder)
Local $title, $edit, $aTabStops = 
$title = "Specify Calculation"
$edit = ControlGetHandle($title,"","Edit1")
If Not @error Then