voltage divider calculations

[gardner]

If you want to use standard values, then you have to use a bit of trial and error to get as close as you can.

And choosing resistors from an E6 or E12 set that match some ideal ratio can be pretty tricky. There are several calculators out there, but this one is my favourite:
http://www.cl-projects.de/projects/tools/resmatch-en.phtml