This subject was somewhat covered in this thread.

The only thing that I would only add to that information is that I've read a lot of calculations were used by professional typists to make sure everything was properly lined up. For example, take this passage from a 1950s typewriting course: "The dimensions of letterhead paper are 8½" x 11". Since there are six single line spaces to the inch, you will seee that the line capacity of letterhead is 66. You should memorize this information since it will be used when claculation vertical placement."

I would make up my own backing sheet. But you also have the option of looking for a typewriter with a bottom-margin marking 'device' on the platen. Usually the left side. I have a Smith Corona Sterling 5 series which has red and green markers and numbers which you can use to work out when you are coming to the end of the page. You set the markers to the relevant size of paper you are using. It's not really a 'device' just clever marks to keep an eye on, but it does work. This manual explains it properly on page 9!
http://site.xavier.edu/polt/typewriters/SC1950s.pdf

Hi Helton

Some of my typewriters have start and finish marks on the left end of the platen roller but others don't, so here's what I do. Take a sheet of 28lb to 32lb white paper for use as your backing paper. Rule in a black line one inch up from the bottom and one inch down from the top on both sides, then rule in a red line 1½" in from the ends. When you're typing with the backing paper in place, you'll see the lines showing through your typing paper. When the red line comes into view, you know you're ½" from the bottom margin and still 1½ from the bottom of the page.

By lining both ends of both sides, the backing paper can be turned over and/or end for end for your next page of typing. This helps prevent the backing paper from developing an unmanageable curl. The distance the black and red lines are drawn from the ends of the backing sheet are entirely up to you, I use 1" and 1½" for my black and red lines as it works for me. You may prefer a 1½" bottom margin (black line) with the red indicator line ¾" or 1" up from the bottom margin, it's purely up to you. Hope this helps and all the best,

Sky 

skywatcher wrote:

Take a sheet of 28lb to 32lb white paper for use as your backing paper.

That is a nice thing to try. I would say "nah, this is too much work", but actually the very manual of the machine suggests the use of a backing paper as a way to preserve the platten and give a better, more homogeneous type print. Adding these lines you mention would give yet another reason to use that paper! Thanks for your answer! (although I still think it is a bit too much work, specially if you are "in the flow", but anyway...)

I found a solution by mistake. I had printed some worksheets for my students and they came out with just one line. Useless as a worksheet, but used as a backing sheet I can see that line through the paper I use. That let's me know when I am almost at the end. But I do like the idea of using a roll of paper.