Nice job with your first vim script!

That said, VIM patch 7.3.787 provides a 'better' solution by replacing the current line's number with the absolute number (because seeing 0 on the current line provides nothing meaningful) IMHO:

  with 'relativenumber' can't see the absolute line number

ftp://ftp.vim.org/pub/vim/patches/7.3/7.3.787