copy a value from a text file and append it to another text file

i need to copy the numeral value from the following string and append it to an existing text file.

i want to include it in my existing batch file trough a CALL function. preferably in a single line or an external file if not other wise possible.

here is the string from which i need the 'average' value:

[Parsed_psnr_0 @ 0000000003893f20] PSNR y:18.250954 u:33.359343 v:36.559083 average:19.962629 min:11.945076 max:41.254674

thanks

i think the solution is here by David Wyatt but i don't know how to get it to work:
https://goo.gl/JS5KQo

Skills: Powershell

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Project ID: #15758998