The security-level describe the trustworthiness of the attached networks. The higher the number, the more trustworthy is the connected network. If inside is your most trusted network, give it 100 (which is default). 0 for outside. If VOIP is nearly as trusted as inside, then 99 is ok. But I still would use 90 so that you later can add am interface between VOIP and inside if you want. DMZ and Public: which is more trustworthy? That one will get the 50, the other the 30.
For your outside-interface you use no subinterface. That's not considered a best practice as you are using the native vlan for that. On a security-device you better should tag all traffic. You should change that to a subinterface as you have done on Ethernet0/3.
Also, again, what about the security-level on the physical interface? Does that interfere with the security-level of the subinterfaces below it?
no, it doesn't matter if it's on the main interface or on a subinterface. The security-level is to classify the firewall-interfaces. Both (on the main interface and on the sub-interface) are firewall-interfaces.