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An option pool expressed as a percentage of post-issue equity has to be grossed up, because the new shares enlarge the denominator. Dividing existing shares by (1 - target pool) gives the fully diluted total, and the difference is the pool — which is why a 12% pool dilutes existing holders by 12%, not less.
Fully diluted total
Total = Shares outstanding / (1 - Target pool %)
Pool shares
Pool = Total - Shares outstanding
Usually existing shareholders, because investors insist the pool sits inside the pre-money valuation. That is the single most negotiated line in a term sheet.
10-15% at Series A is typical, sized bottom-up from the hires planned before the next round rather than from a rule of thumb.