Linux flavours vary slightly when netbooting, here are the two most common versions I use.

default syslinux/menu.c32
prompt 0

MENU TITLE Linux Mint Installer Menu
menu separator

Label Linux - Mint 19.0 amd64
KERNEL mint/19.0/vmlinuz
APPEND initrd=mint/19.0/initrd.lz boot=casper locale=en_GB netboot=nfs nfsroot=192.168.1.111:/export/tftp/mint/19.0/iso toram systemd.mask=dev-hugepages.mount systemd.mask=dev-mqueue.mount systemd.mask=sys-fs-fuse-connections.mount systemd.mask=sys-kernel-config.mount systemd.mask=sys-kernel-debug.mount systemd.mask=tmp.mount

Label Linux - Mint 18.3 amd64
KERNEL mint/18.3/vmlinuz
APPEND initrd=mint/18.3/initrd.lz root=/dev/nfs boot=casper netboot=nfs nfsroot=192.168.1.111:/export/tftp/mint/18.3/iso nosplash --

LABEL Return
MENU LABEL ^Return to Main Menu
KERNEL menu.c32
APPEND pxelinux.cfg/default

There are differences between Mint 18.x and 19.x - here is a summary:

• KERNEL - Path to vmlinuz kernel image, as found at [root_of_mint_iso]/casper/vmlinuz
• initrd - Path to initrd.lz root filesystem image, as found at [root_of_mint_iso]/casper/initrd.lz
• nfsroot - NFS path to the extracted Mint 18.x iso image