Read more on value of control and the illiquidity discount

Spreadsheet
Value of control
Illiquidity Discount

If you do a valuation right, there should be no need to apply discounts and premiums for most items to the estimated value. Consider the widely applied private company discount in the valuation of publicly traded companies. The rationale is that discounted cashflow valuations assume that a firm is optimally managed and most firms are not. This is patently absurd since the analyst chooses the inputs that go into the discounted cashflow valuation. If a firm is poorly managed with a sub-optimal debt ratio and a low return on capital, the discounted cashflow valuation with these inputs will already reflect the poor management. Consider also the premium that is often applied for control. To value control, all you would need to do is re-value the firm with optimal management and the difference between this value and the status quo value should be the value of control.

Liquidity is a tougher problem. All investments are illiquid, but to varying degrees; for publicly traded firms, it takes the form of a bid-ask spread and for private firms it takes the form of a discount on estimated value. The key is to be discriminating. Not all private companies are equally illiquid. Applying a rule of thumb (25-30% is widely used) strikes us as inappropriate.