ALTER table - how much free space is needed

In attempting to unsuccessfully alter some large Informix tables, we've run into failures. The question arises, is how much free space is needed, as indicated by tbcheck -pe? Specifically, if my first allocation was 10,000 blocks, and the next size is 1,000 blocks, will Informix require a section of 10,000 blocks to do the alter in??? What is the first next section were adjacent to the original 10,000 - now, I believe, I have 11,000 blocks - the initial allocation is not stored by Informix - so I presume that I would need 11,000 blocks??? Thanks for any info, Ihor Kinal att!trumpet!ijk