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 | 0Â Â Â Â Â Â |
| 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.