>> ..could this be as simple as:
>> address 192.168.0.199/24 #???

Yes

> No, in /etc/network/interfaces it needs a net mask line like this :
>

>> auto eth0
>> iface eth0 inet static
>>    address 192.168.nnn.nnn
>>    netmask 255.255.255.0
>>    gateway 192.168.nnn.nnn

No

RTFM?

Documentation states, for both INET & INET6 families:
address address
        Address (dotted quad/netmask) required

netmask mask
        Netmask (dotted quad or number of bits) deprecated

Are we really debating how to configure network addresses without first
searching in the man pages?
I hope no-one is giving speeches in the whole lot :oP

The only things which are not documented in the "interfaces" man page is
how the default value for optional directives is computed. In doubt,
specify those, but they are technically not required. Welcome to
"default values" wonderland.

Happy documentation reading,
Bernard (Beer) Rosset
https://rosset.net/