Anyway to navigate percentage-wise in vim?

Sorry for a short answer, but

just type 50%

                                                        *N%*
{count}%                Go to {count} percentage in the file, on the first
                        non-blank in the line |linewise|.  To compute the new 
                        line number this formula is used:
                            ({count} * number-of-lines + 99) / 100
                        See also 'startofline' option.  {not in Vi}

There's the [count]% command (how intuitive!) for absolute navigation. For relative navigation, you have to calculate the amount yourself. E.g. to scroll down 33%:

:exe 'normal!' 33 * line('$') / 100 . 'j'

(You probably want to create mappings that use v:count for these.)