There are two types of text layout in FCPX, plain/basic — like you get with the Basic Title (and about 90% of all others) and a "Paragraph" layout, which is essentially a stripped down version of TextEdit embedded into FCPX. The paragraph layout allows for critical arrangement easily with Tab Stops and "bounds" control.
You can find one of them in Titles > Lower Thirds > Upper. It's an absolutely simple, stripped down title. No graphics.
The default arrangement is as shown above. To make the Ruler tool appear, click the small blue rectangle at the upper corner of the viewer (you can turn off the ruler by clicking it again). You can right click on the ruler to set Tabs (based on the shown selections.) You can move tabs just as in TextEdit by click-dragging them to position. (Make sure All Text is selected that you need the tabs to apply to.) When you click and drag on a tab, you will get accurate position feedback above the tab. Zoom in for better precision. To remove a tab, simply drag it off the ruler.
The default for *this* LT is to have the paragraph bounds align the top of the *text* to Text Safe Zones. To adjust the bounds to whatever you require, grab the small square controls along the bounds edges. If you simply select the text without the ruler and there are blue dots along the corners and edges, you will stretch the text area and introduce scaling distortion to the region (there is a difference!) Text will not go beyond the left-right bounds of this layout type.
For a credits screen, you could grab the square on the bottom line of the paragraph layout control down to the bottom of the screen (and beyond if necessary — you can animate the entire pane with the transform parameters.) However, in the default configuration, text will flow down below the bottom of the bounds as needed. If you run into "irregularities", size the bounds to accommodate your text.
If you have Motion, you can create one of these with a most useful attribute: that of Auto-Shrink. Auto-shrink will automatically adjust the text size on the screen to fit within the bounds. It can also be designed to "compact" text by narrowing the characters if longer lines are used (caveat - all text is resized to match whatever is necessary on the longest line.) The use of "compact" text will be dependent on whether auto-shrink To All Margins is set, in which case the text is simply sized smaller.
At any rate, you could set tabs for each line and accurately arrange tab stops to help you align the text. If you have Motion, and you want to modify this lower third to accommodate Auto-Shrink, right click on the LT in the Titles browser and Open a Copy in Motion. Select the text layer and in the Inspector > Text > Layout > Layout Controls, simply right click on Auto-Shrink parameter and select Publish from the drop down menu. Save. That parameter will now be available to you in FCPX.
HTH
