Actually upon further research I was mistaken! It turns out that crit chance in Gen I is based soly on the user's species' base Speed, not the individual Speed stat. For Red and Blue, the formula is baseSpeed / 512. In Stadium, it's baseSpeed + 76 / 1024. Moves with a high critical hit ratio (such as Slash and Razor Leaf) apply an 8x multiplier to this chance, though it's capped at 255/256 (or roughly 99.6%). Notably, in Red/Blue, any Pokemon with a base speed of 64 or above will cap this.