For WU completed successfully and sent after preferred deadline but before final deadline, PPD appears to increase linearly as TPF decreases.
There is a large jump in PPD (because of bonus points) for submission at the preferred deadline boundary. After that, PPD increases not linearly but with a 'power curve' as TPF decreases.
http://s14.postimage.org/ncp6i2t1t/PPD_vs_TPF.jpg
EDIT: OOPS!
