I tried to modify the function to be more similar to factorio's. feel free to use or not.

function number_to_short_notation(number)
  if number == nil then
    return 0
  elseif number < 1e3 and number > -1e3 then
    return number
  end
  for _, v in ipairs({"k", "M", "G"}) do
    number = number / 1e3
    if number < 10 and number > -10 then
      local trunc = math.modf(number * 10);
      return string.format("%.1f%s", trunc / 10, v)
    elseif number < 1000 and number > -1000 then
      local trunc = math.modf(number);
      return string.format("%.0f%s", trunc, v)
    end
  end
  -- factorio number should be up to 2.1G, but failsafe
  return string.format("%.0fG", number)
end

pros: fits to more narrow space, consistent with factorio's.
cons: I didn't notice by myself, but maybe a bit slower because it has more conditions.