If you use the 'right align' tag, you can get the figures lined up. (See for advanced code options, or just use the icons above the composition box.)
Rent allowed 10,000
Tax 4,200
However, if you're a perfectionist, the text still looks a little messy because the whole line is right-aligned, not just the figures, so the text is misaligned.
You could correct this by using a fixed width font such as courier and using a character such as a full stop as a spacer, with a pale font that won't show up too badly.
For example:
Rent allowed 10,000
.........Tax. 4,200