That's a good question, Tim.
I started trading the S&P's in 1982, just a week or so after they were introduced in April of that year. You have to keep in mind that then the S&P was trading in the low 100's. Not only that, margin requirements were similar to commodities. So if I remember correctly, you could trade 1 contract, at 500x the index (nominal value about $60,000 if the index is at 120) by putting up 2000 or so. So in those days 1 unit for me would be 1 contract per ten thousand dollars of account size.
But then along came the 1987 crash. Margin requirements went up dramatically, so when the index was trading around 300, at 500x, nominal value about 150k, you had to put up 10k, sometimes a little less, sometimes more. If margin were 10k, then if I remember correctly I would trade one unit, one contract, per 30k of account equity.
You can see the basic idea. I want to trade 1 contract = 1 unit per roughly 3 or 4 times the overnight margin.
This leaves room for a second contract carried overnight plus a reasonable drawdown.