Just to verify the expected math:
chamber_block_minimum = (floor(reactor_block_count/200)+1) * 25;
This yields a minimum of 25 blocks per chamber (kinda hefty if you want a chamber in a light craft), with 250 blocks for a reactor that falls between 1800 and 1999 blocks.
Personally, to accommodate builders of smaller ships (raises hand), I'd recommend a more granular algorithm.
chamber_block_minimum = (floor(reactor_block_count/40)+1) * 5;
This yields a minimum of 5 blocks per chamber (still kinda hefty for a light craft, but more possible), with 250 blocks of chamber for a reactor that falls between 1960 and 1999 blocks.
If you wanted to be supremely granular, you could grow a chamber block requirement one block at a time.
chamber_block_minimum = (floor(reactor_block_count/8) + 1);
This yields a minimum of 1 block per chamber for every 8 blocks of reactor (7, 15, 23, 31, etc....), up to 250 blocks of chamber for a reactor between 1992 and 1999 blocks.