what I'd do is pretty much what Rayn suggestes. I'd add that you can also use a overline as well as an underline for the answer line to make the answer be between equals lines. You find that in the borders options in the paragraph section of the Home tab, just after the alignment buttons, First select the answer line. Then it's easiest to use the shortcut keystroke Alt+h, b. In the dialog box you can choose exactly which borders you want. As you select a border the dialog box closes so after you press the bottom border button you come back out of the dialog and are on the answer line of your calculation. Then you go back into the borders dialog using Alt+h, b and then tab to the top border. As you tab to the top border you'll be able to hear that the bottom border is ticked.
The other thing I'd recommend is using a monospaced font like Courier for your math parts. That way you know that each digit occupies the same amount of horizontal space, with a 1 being placed in the same physical space as a number 8, which is similar to the idea of doing it in Braille because each character occupies the same physical space and you know the numbers are lined up under each other.
With a monospace font, you could type:
That way you get the plus symbol on the same line as the 7, but at the far left of the calculation. There is then a space before any numbers, and then you have the 123 and 456 lines and then you need extra spacing to put the final line, the number 7, into the right hand column of numbers.
It'll probably drive you crazy keeping track of how many spaces you need, but in practice you can start in the top line and just count how many times you hear "space" as you right arrow along the line before you get to a number. You're result should be:
Hope that helps, or hope that all adds up, I should probably say ;)