1. Look at the day after a previous earnings release which has a similar vol crush to what you expect.
2. Using the option chain from that day, look at the calendars with similar time frames to the calendar you want to analyze.
3. Move away from the 'money' in both directions on the option chain to see how far you can go before the value of the calendar drops below the price of the calendar you're analyzing.
4. That will give you a pretty good idea of how much movement your trade can withstand, and what the value of the trade will be depending on how much (or little) movement the stock has.
How did this trade turn out? Did the IV in the longs in the back months hold up compared to the short months? I have been looking at a very similar strategy of finding candidates with juiced up front month and setting up double calendars selling the front month with the goal of the front month IV crashing more than the back month IV.