We use Scalar. They are fine. This is a cya exercise; it has no meaning apart from the SEC. Investors certainly don't care about their opinion of stock value. You should not be stressing about this at your stage. You should not be spending money on this. Don't go talking to a valuation firm's salesperson - their job is to sell you a valuation.
The way you would get "in trouble" is that, years from now, someone at the SEC (not even IRS!) would come back and try to tell your option-holder they owe a small amount of back taxes because the discount you set was not quite right, years before liquidity. I've never heard of this happening. I'd expect you'd have to royally and personally piss someone off to have anyone look at this and even then, the only situation your lawyer couldn't get you out of would likely be if you set the strike price to $0. When you raise your next multimillion dollar round, spend a few grand to get someone else to set the discount at that point (and only at that point).
Typically, a company like Scalar accepts the preferred price investors paid as the true value of preferred and then calculates the discount of common. We didn't use them for our priced seed round, but just set the common strike price as 10% of our preferred price. Years later, at our series A, Scalar recommended we increase it to around 20-25% of preferred, if I remember correctly.
Do you not have a concept of preferred stock yet because you have not had a priced round? Find a friend who you think is or was at a similar stage who did a priced round and use his or her pre-money valuation. If there's some reason you know why you couldn't get an investment right now, add a discount for that fact. Divide by the total number of shares on a fully diluted basis (i.e. current shares outstanding plus warrants plus option pool) - there's a good faith swag at the preferred price per share. Apply a very aggressive discount (minimum 80-90%). There is a good faith estimate of your common strike price. Go get a beer: you've thought about this too much already.
Also, just as an aside, if you haven't had a priced round yet, you're not vested: you're the owner. A vesting schedule is meaningless without partners: when you do a priced round, expect the investors to ask you to start a new vesting schedule (and do not object: that's a reasonable request; negotiate on how much you need to vest - try for 75-80%).