How do I fix 425 MB free after deleting startup disk to reinstall El Capitan?

You need to show the complete Disk Utility....you've hidden a critical part for understanding the drive layout. I think you have only single partition which is good.

And what is the exact model of your Mac? Is macOS 10.11 El Capitan the highest version of macOS compatible with this Mac (very important information to have for us). If El Capitan is the most recent version of macOS compatible with your Mac, then the 425MB is expected because it would be part of the local recovery partition needed for reinstalling macOS.

If macOS El Capitan is the most recent version of macOS compatible with your Mac, then you either need to use an OSX 10.6 Snow Leopard upgrade DVD installer, or a gray recovery DVD (10.6.x Snow Leopard) which originally shipped with the Mac from the factory....which one depends on the exact model of your Mac (very critical info).

If you have access to another Mac from generally from 2007 to 2015, then you can use it to create a bootable macOS 10.11 El Capitan USB installer using the instructions in the following Apple article:

Create a bootable installer for macOS - Apple Support

If you have access to either of these alternative installers, then I would advise erasing the whole physical drive instead of just the "Macintosh HD" volume so that a new partition table is created. How you do this depends on whether you are using an El Capitan installer or a Snow Leopard installer. At the moment you do not want to erase the whole physical drive since that would destroy your local recovery partition which is the only thing allowing you to boot this Mac at the moment.

Why did you want to reinstall macOS? That same issue could be why you are having problems reinstalling macOS now. I suspect your internal Hard Drive is failing & is probably why you were having issues before & now.

You can try installing macOS onto an external USB3 Hard Drive or SSD to see if that makes any difference. I think that is the best way forward at the moment as it will provide you & us with lots more information about what is going on with your Mac, plus it gives you more options going forward with the troubleshooting.

I don't quite get what you have previously done. But try to boot to local Recovery via "Command-R" or if that fails to Internet Recovery via "Option-Command-R" (that might take a while like 30 minutes). Then try to install macOS to the internal (or external disk).