The premium is measured from the undisturbed price, meaning the price before rumours leaked. If the stock already ran up 15% on speculation, a 30% headline premium is a much smaller premium to the day-before close.
Premium size signals a lot. Very thin premiums invite shareholder lawsuits and a rival bid; very large ones raise questions about the acquirer's discipline and often knock the acquirer's stock down.
Example: a target closed at $28 a month before rumours, drifted to $33, then received a $38 offer. Against the undisturbed price that is 36%; against the last close it is 15%.
Related: acquisition, merger, hostile-takeover, merger-arbitrage