I would simply setup and scape the tank, let the plants go through transition and grow in. From there forget the lenght per volume rule. Because it's not only ancient, it's commercialy based guidline. As said above by
@Edvet volume says nothing about how well the setup can coop with the organic pressure and maintain water quality.
Bottom line every fish tank no matter how it's setup up it's a Diorama, a little "Art"ificial under water world the way you like to look at it. Than rather use proper scaling and how well the fish you choose compliment the setup and still live a happy active live. Not only looks and size but also behaivor and compatibilty. Not only compatibility between the fish spp. themselfs but also towards the biological capacity of the setup and how much maintenance are you able to perform on a weekly or even daily basis.
Review questions regarding the fish, what does it like/requires?.. Diet? Light penetration, Shelter and how should thise be provided, plants or caves? Free swiming space and where does it like to swim, lower, middle or upper region? Is it an active or a passive swimmer, does it school, schoal, pair up or is it rather solitairy. What is the teritorial demand/behaivor among eachother and others. Is it an closed or open top setup, does the fish like to jump? Than this fish is not suitable for an open top. Etc etc.
Than if you have a desired fish spp. in mind before you start setting up a tank. Than research the fish and build a tank that fits their requirments. Than if you don't know, than setup up a tank, let it grow in and decide from there what compliments this scape the best.
Than when it comes to size, there is no actual rule.. It's simply common sense and how you like to see it personaly.
My personal take on it is, a fish needs traveling space and broken lines of sight. To give it this sufficiently in make sure it can swim it's own lenght at least 20x traveling from left to right and 10x from front to back. My tank is 35x35x90cm thus my limit is it should not be larger than 35mm, preferably smaller. It not only scales beter into the dimension it also looks happier and more natural.
I know people that use the 1cm/L formula and put a 12cm solitairy fish in a 80x35x35 cm tank and proudly and happily present it like that. They don't think for themself only live up to the given rules from abook and think its ok. But don't see the fact that this fish wiggles its tail 3 times and than bumps its nose into the next glaspanel and needs to turn 180°. If it gets excited it realy bumps into all 4 panels in less than 2 seconds. For me personlay, it's a fish in a jail cell punished for life.
I feel sorry for it.