I found the formula on the Smogon Manipulation Guide Part 2 - How to RNG a Perfect Pokemon. It could be outdated though. when u mean a block of seeds you mean for example frames 64,65,66? where anyone of those frames would suffice?
There isn't a block of them until around 500+ . Iv been trying for frame 62 or 69 for ages, have been hitting the desire IVs several times but no luck with the nature.