I'm trying to value a company's equity as a call option as the principal amount of outstanding debt exceeds the total enterprise value of the company.As this is a private company, I have found publicly traded comparables and calculated their historical equity volatilities (they don't have publicly traded options). However, as I need volatility of the enterprise, I need to adjust these observed equity vols.I saw a formula by Schwert: vol of enterprise = (S / V) * vol of equity, where S is the total equity value and V is the total enterprise value.I'd appreciate your thoughts on any and all of the following questions:1. In calculating S / V, should this be based on current values? Or an average of historical values since my equity vol is historical?2. Same as question 1, except assume that I was able to get implied equity vols for comparable companies.3. Are there different formulae (different from Schwert) you would suggest I consider using? If so, which ones (links to relevant papers would be appreciated).Thanks!