At the time a client requests an IP, they don't have one, thus the "any" as source. If you put in your source of a specific network it will fail since the user is not on a network yet. What are you trying to accomplish by changing this?
Typical setups include the following DHCP related rules
user any udp 68 deny (to deny a client from being a DHCP server)
any any svc-dhcp permit (to allow all clients to request and get IPs)