Hello Gringo
First off, understand that, in the historical sense of the word, you are already very diversified. Diversity used to be used to urge investers to buy stocks in multiple sectors. Today, if you own just the SP500 you shares of 500 different companies across virtually every market sector. It's estimated that it alone has 80% market coverage. Adding to that, many of the largest companies in the SP500 make a large portion (often the majority) of their income outside the US (think Apple, Coke, GM, McDonalds, etc. etc.) In that regard over a quarter of all profits from holding the SP500 come internationally. In a sense you already have far more than your estimated 20.38% invested 'internationally', even though you are holding a lot of US companies.
A better question to ask yourself is how do you want your money invested? Historically US Small-cap stocks have had greater volatility and slightly better returns that US large-cap stocks. OTOH, Large-cap stocks pay larger dividends. Who knows what will play out in the future.
There's certainly a lot more potential for growth in emerging markets, but many of these markets are fraught with political turmoil and instability. You could invest heavily in stocks from country X only to have a coup seize power of all private industry. It's happened before, and it will happen again... the question is just where and when.
Personally, I don't think you are overweight in US stocks. You've got some dedicated international funds and some additional international exposure by holding large internatinoal US companies in your SP500. But if you feel you want to hold more international stocks, by all means do so - just consider the fees first (which will likely be higher) and know that the less stable the country, the greater the risks.
EDIT: what you aren't doing currently is diversifying outside of stocks. Historically stocks have generated the best yields over time periods >12 years in virtually all cases. It's where I park 90%+ of my savings. But diversifying also can include bonds, gold, REITs (of which you have a small percentage), and various currencies. Each can do something slightly different in different market conditions.