Find a movie I think is comparable (the usual date of release, target audience, relative competition, etc) and use their weekend to weekend % drops as a starting point that I'll tweak based on how I like a movie.
Then I work the weekdays backwards based on how movies do on weekdays this year from the numbers I get doing the above step. Generally readjust my guess the first Tuesday after OW for obvious reasons (first Monday #s and stock price settling if its not that obvious).
Generally only do that on stuff I think can be a decent post OW play though (looking good on CAPAM, CBALN, RSAPE....taking a bath on SMURF....could go either way on CHNUP, FWBFT)