To optimize the key_buffer_size, you need to review the key_read statistics:

MariaDB [(none)]> SHOW STATUS LIKE “key%”;
+————————+———+
| Variable_name          | Value   |
+————————+———+
| Key_blocks_not_flushed | 0       |
| Key_blocks_unused      | 1428301 |
| Key_blocks_used        | 4187    |
| Key_blocks_warm        | 0       |
| Key_read_requests      | 82847   |
| Key_reads              | 4187    |
| Key_write_requests     | 76      |
| Key_writes             | 76      |
+————————+———+
8 rows in set (0.00 sec)

Key_read_requests should be at a ratio of 100:1 or 1000:1 to key_reads.