In general, I find that about 30% in non-US stocks is about right to get the full diversification benefit of international investing (lower risk, higher returns than US-only allocation). "Just" also helps behaviorally as it can be tough for many investors to hold significant non-US allocations during periods when our domestic market is doing well comparatively. If one lived in Canada or Australia where their home market was a very small percentage of the world market (<10%), it might make sense to up that to 50%, but for US investors I don't think its necessary. Further, don't forget that your future liabilities will be tied to the US economy and US $s, so that is another reason to hold a high % of US stocks.
Within the international allocation, I've found that about 2/3 developed and 1/3 emerging is a good split. EM is higher risk (and higher expected return), but offers a bit more diversification than non-US developed.
If I were using Vanguard funds, I'd stick with Vanguard Int'l Value, Vanguard Int'l Explorer, and Vanguard Emerging Markets Index (3x33). Another variation would be 17% Vanguard Int'l Value, 67% Vanguard World exUS Small Cap Index, 17% Vanguard Emerging Market Index. Those wanting exclusively index funds can replace VTRIX with iShares EAFE Value in either case. You also get a bit better diversification benefits by holding non-US small and value stocks as opposed to non-US market portfolios like EAFE. In doing so, you don't need as much Int'l % to get the overall benefit.