For my thesis, I'm trying to calculate implied correlation values from bivariate options. I train my model on 10 years of returns data, price the options, and then invert Stulz's Formula (basically Black-Scholes for bivariate options) to find the correlation coefficient given the price. However, Stulz's formula also requires inputs for the asset volatilities. I've tried using the implied volatilities, but they change too quickly relative to the prices from the model to be used. I have the same problem for historical volatility, but also its sort of unclear what time window I should be using. I was trying to think of alternatives for those, but I'm drawing a blank. Are those the only logical things that I could use for asset volatilities? If so, the issue likely lies with my model. Thanks!