As long as the table is not updated /inserted frequently, bit map index is OK.

If your result set is going to be 12% of the rows from the 500,000 rows table, then I would not allow CBO to use neither bit map index nor the B-Tree Index, because it requires more I/O for reading Index and data blocks to figure out the result set. In this case I would choose full table scan, because the data is going to be present in LRU for longer time.