Mongo Chunk 250000 document limit is creating problem in chunk migration

We can set chunk size limit for a DB. But chunk has a limit of 250000 documents per chunk. My chunk size is 250 MB which varies the number of document in each collections. So normally some of my collections crosses this limit of maximum documents. Could you please suggest how should i do the calculation in this case.

I see in my primary shard it alsways show different chunk size for me with each collection even i have set it for 250 MB some times it is shown as 125 MB and some times it is shown as 64 MB in one case it was 1 KB only and it caused a real trouble on my system.