The standard practice is to give equity based on the percentage of fully diluted shares. However, percentages can't be included on offer letters because the number of shares at a company change over time.
Pulley includes an % Shares Calculator to make the conversion between % and number of shares easy.
What's included in the denominator for fully diluted?
Fully diluted includes the entire option pool (assuming it's granted) and all shares (preferred & common). Fully diluted does not include equity given to investors in SAFEs because the SAFEs is have not converted. For SAFEs without caps or on a discount, it's impossible to know until the priced round valuation how many total shares will be fully diluted.
Click on % Shares Calculator to try Pulley's calculator.