GTM "static persist mode"

Trying to resolve a way to GTM load balance between two pool members using "static persist mode" with the LDNS source IP address. I have the static persist CIDR mask set to 24 bits but the issue is for larger ISP's like Comcast, this does not seem to work since the network space they have is larger than this and it appears LDNS requests from end users are load balanced outside of the CIDR mask (those 254 hosts). Does anyone know of an effective way to employ the "static persist mode" for a condition like this?