There really is no big mystery about the cost basis of stock acquired via SARs or RSUs. What makes the reporting of sales of shares acquired in this fashion somewhat difficult is that on the 1099-B itself brokers are only required to report your "out of pocket" costs for these stocks, which typically is $0. That basis is wrong and if you simply enter the 1099-B as it reads and don't adjust the cost basis you end up reporting the same income twice. (However, brokers are supposed to provide along with the 1099-B a "statement" of some sort that does provide the correct cost basis information.)
It's best to think about sales of these stock on a "per share" basis; that makes it easy to come up with the correct basis when reporting the sale of less than the entire grant, as in a sell to cover situation. The per share basis of these stocks is the same as the per share "fair market value" used by the employer to calculate the compensation associated with receiving the shares. That is, (GROSS number of shares received before "withhold" or "sale") x (per share FMV) = Compensation associated with that lot.
Presumably all the necessary taxes associated with the shares were included on the various "taxes" boxes of your W-2 and, generally, a "same day" sale of stock results in a small loss due to selling commissions and fees.
