A simple burn down chart might do the trick.
We'll side step estimating and the value of it right now, that's a longer term thing that will help in the long run.
Instead we'll go with fixed time as it sounds like you have a really solid grasp of the work you have to do.
Step 1: Determine the number of workable hours remaining in your project. This is your Capacity.
Step 2: Determine the total amount of work to be done. This is three sub-tasks.
2A- New Copy: For this, use hours as best as you can manage. The key is to well understand the "New Copy" effort. For example say you know that roughly 500 words of new copy takes an hour to implement, divide all the new copy by 500 and that's the number of hours for New Copy.
Now divide the amount of done work by the new copy work. This gives you a percentage of change. This can be used to project how many hours of new copy you can expect to come from the undone work.
2B- Estimate your upcoming copy edits. Look at the work you have already done, determine how much copy you have put in and how much copy edit has been asked for. This is a percentage of change. Apply that to your undone work. Then figure out how long it takes to fix X amount of copy. For example 1000 words per hour.
2C- Determine how much time is left for new functionality/ pages.
Step 3: Add up all of Step 2 and determine how many hours of work there is right now. Then add in the projected work for copy edits and new copy. This is your projected work.
Do a simple graph with the vertical axis showing two values, total work to do and total work to be done. The horizontal is a date line. Now chart your progress based on the number of working hours per day. Say you can do six hours of work a day, each day the to do is reduced by six hours. Project this line out and see if you will be done on the due date. Do the same for project work to be done.
Step 4: I'm going to assume you're already not going to make your date, much less be able to take new copy. Put all the remaining new development, current copy edit, current new copy into a single list. If you can meet with the client face to face use 3x5 cards if not use a spreadsheet.
Step 5: Show the customer the burn down. Show them how the date won't be made (or won't be made if new work is added). Ask them what they want to do. When they say "What should we do?" pull out the list of work to be done and have them prioritize it in rank order (nothing can be equal).
Feel free to ping me direct if you have more questions.
