Set the letter's value in the Refers To box. You can create a value from data in a cell in the worksheet or you can add your own number. By default, Excel puts a reference to the currently selected cell in the box. If this contains your value, keep the default.
Gravely (the adverb) comes from the adjective, which comes from Latin (gravis), where it meant heavy or serious (this is where we get gravity--heaviness). Gravely ill means seriously ill, not soon to be in the grave. (and proof [1]). Bonus tidbit: grave (the noun) comes from words that originally meant to dig then a place dug, as for burial or, a grave. This is the same root as engrave.
Fatal, deadly, lethal, mortal apply to something that has caused or is capable of causing death. Fatal may refer to either the future or the past; in either case, it emphasizes inevitability and the inescapableâthe disastrous, whether death or dire misfortune: The accident was fatal. Such a mistake would be fatal. Deadly looks to the future, and suggests that which is likely to cause death (though not inevitably so): a deadly poison, disease. Like deadly, lethal looks to the future but, like many other words of Latin origin, suggests a more technical usage: a lethal dose; a gas that is lethal.