Offer premium
The offer price minus the unaffected share price, divided by the unaffected price. A negative figure is a discount to the close from before the news leaked.
The denominator has to be the price that does not already contain the deal. Once a rumour is in the tape, the last trade is a blend of the old price and the offer, so the premium against that trade looks smaller than it is.
The spread per share is a dollar figure. The premium is a percent. Synergies and control are dollars. Mixing a percent premium with a dollar synergy claim without converting is how a bid gets misread.
The offer premium calculator divides the two prices. Enterprise value is the next object if the bid is for the operations.