Hi chesatoshi, sure I use a script called =ImportJSON() that you can import into Google Spreadsheets. Here's the github page:
It requires a bit of programming knowledge but if you find it overwhelming don't worry, I'll publish updates to the spreadsheet regularly here. Actually, if you rebalance too often you are doing it wrong. You shouldn't be looking at the indexes too often, this is something very important I learned from reading Ben Graham and Warren Buffet's books.