Function plan 

fn plan(n_prompt: usize, k: usize) -> Plan

The card’s ceiling is 1.3x the input plus 32, taken over the whole prompt rather than the transcript, so it sits looser than the figure it comes from.

n_ctx covers the highest position ever written: the prompt, then at most budget emitted tokens, then a whole k-token draft written speculatively past the last accepted one. That highest index is n_prompt + budget + k - 1, so the count needed is one more than that and this leaves exactly one slot spare.

A batch holds either the whole prompt on the first decode or one confirmed token plus a full draft on every later one, so it takes the larger.