OK, I've run a statistical analysis on optimally hedging YM exposure with an offsetting ES position, on an intraday basis.
Time frame: last 10 trading days, 2/22 - 3/7/05, RTH.
Frequency: 1) hourly bars; 2) daily bars.
Here's a snapshot (done with SnagIt) of the relevant area of the Excel spreadsheet:
The results are not that different over this particular time frame, whether using hourly or daily data. However, my recommendation for "emergency" intraday hedging (like today) would always be to use hourly results. So, that means going 0.83 ES contracts long (short) for each YM contract short (long).
Steve, if your standard YM trade size is a 15-lot, 15 x 0.83 = 12.5. So you'd want to hedge with either 12 or 13 ES contracts, your choice. That's the "pure", optimal hedge itself. Anything beyond that would be taking a separate, directional bet, per your trading plan, using ES as a proxy for the unavailable YM.
Of course, this works in reverse as well. Should the CME GLOBEX go down, um, in flames and stay down for a while, a YM position could be used -- if one wants to stick with futures -- to hedge, e.g., ES exposure, with a reverse 1 / 0.83 = 1.2 ratio (1.2 YM for each ES).
I'll gladly answer any questions about the above, if any.