What is the equation for Time Machine required space?

Once again Time Machine says it has run out of room to do my backup. I guess I just don't understand the equation for figuring this out.
These are my specs:
Disks being backed up - 2 drives
          Internal Drive - 640gb, 482gb in use
          External Drive - 500gb, 105gb in use
     587gb total data in use
Time Machine disk - 1TB, 715gb in use, 284gb available, asking for 303gb
TM wants space that is more than half of my total data in use. I have one (old) 2gb file, all others are 99mb and smaller, mostly much smaller.
Why isn't TM removing older backup files to make room for the latest? Why does it think it needs a block of 303gb when the new files it needs to back up are all in the sub 10mb range?
On the surface it seems like TM is using disk sizes to do it's caluclations instead of file size.
What is the equation it uses to figure this out? I am puzzled...

jlmcvay wrote:
Starting pre-backup thinning: 286.19 GB requested (including padding), 265.05 GB available
That's your main problem; wanting to back up about 238 GB (plus 20%). Curiously, that's a bit less than the previous messages.
See what's shown for Estimated size of full backup under the exclusions box in Time Machine Preferences > Options. According to the figures in your first post, it should be about 587 GB. If it's considerably larger, then there are two possibilities:
Something else is connected, and Time Machine is trying to back it up.
There's a directory problem on one of the disks being backed-up, so Time Machine thinks it's much larger. If so, try verifying your internal HD, and repairing the external, per #A4 in Time Machine - Troubleshooting.
But if the estimate is reasonably close, then something very large has changed. See #D4 in Time Machine - Troubleshooting for some of the common culprits. Unfortunately, there's no way to tell in advance what it is; you can only see that after a backup completes.
If nothing there helps, try excluding the external HD and trying again (per #10 in Time Machine - Frequently Asked Questions) -- that should tell us whether the problem is on the internal or the external. Then we can try to narrow it down.
Seems like there is a problem with the Event store UUIDs for my internal drive.
Not a major one. That message means that Time Machine isn't sure that the previous backup was completed normally (which it obviously didn't). When that happens, Time Machine can't use the Event store, but has to do a "deep traversal" to compare everything on the drive to the backups to figure out what's changed and needs to be backed-up. That takes a while, of course, and means your backup will take considerably longer than usual.
I commonly have iPhoto open all the time. If TM cannot back up iPhoto while it open that would explain why it needs to backup a lot of data.
That's a possible explanation, but 238 GB is a lot of photos. How large is your entire iPhoto Library?
Time Machine can sometimes back the iPhoto Library up when iPhoto is open -- it seems to depend on just what you've been doing. So it's fairly unusual for it to be missed for very long. (It ought to send a message after a certain time, but it doesn't.)
If excluding the external HD doesn't turn up a clue, try excluding your Pictures folder.

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☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
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☞ The log contains a vast amount of information, almost all of which is irrelevant to solving any particular problem. When posting a log extract, be selective. Don't post more than is requested.
Please don't indiscriminately dump thousands of lines from the log into this discussion.
Please don't post screenshots of log messages—post the text.
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What is the equation for Time Machine required space?

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