Question: "At what volume should I mix the bass instrument at an instrumental?"
Instrumental means the backing instruments of the song.
Firstly well then, i'll manage the concept of percentage. It does not work well all of that well for audio. In particular, consider a proof at a very good, healthy, yet not ear-damaging level. Reduce it by 6 dB. You may clearly notice the visible difference, but the amount is still strong. Try another 6 dB. It has recently gone down by the same amount, yet is still very clearly audible. Another 6 dB. It has faraway from gone away yet.