Generally we tell people starting with CO2 that they should get 30 ppm of CO2 into the water - that's what the plants need. This is not good advice!
Tropica has a pretty good, brief article on this subject: Fertiliser and CO2 for your aquarium. - Tropica Aquarium Plants
This article says "A bit of CO2 (e.g. 3-5 mg per L) is better than nothing. Plants that are marked "Medium" require about 10-15 mg CO2 per L, but "Advanced" plants require 15-30 mg CO2 per L."
I am not trying to be contrarian, nor defend/attack anything or anybody. I just want to provide a different perspective. All the respect Hoppy for your experience, knowledge and interesting comments and topics.
I feel we could argue the same for every nutrient recommended level. Few aquariums need
3ppm PO4 weekly. Furthermore, what one aquarium needs
today might not be the same as tomorrow or after a big trim. I will still recommend this level, provide it all the time as there is nothing wrong with a little extra with the 50% wc. Always changing it increases the likelihood of running into deficiencies.
Then, the word need
is not very precise. My aquarium can survive with 2ppm PO4, but it does a lot better with 4ppm PO4. Which level is needed ?
I use PO4 for my arguments as CO2 is even more problematic. I think most of us had aquariums where the KH/pH chart gave us large overestimation of our CO2 levels, other where the indicator turned yellow but algae/plant problems were solved with extra CO2. If your measuring tool says you have 30ppm, add a very large confidence interval around it.
CO2 is also very variable at one time-point in different places of aquarium. Water that has 60ppm at the exit from the reactor could end up with 15ppm at filter intake after passing through plants and surface layer. How, when, where is the measurement to be done to get an accurate estimation of 30ppm is often a problem for newcomers.
As you rightly point out, light is the main driver of CO2 demand. However things are not as easy as only looking at light levels at the ground. Plants' needs
also vary across the height of the tank... shaded bottom layer is ok with less, the nearer the light the more CO2 needed. Same light and aquarium, one aquascape can have lower CO2 as it is mostly grass; the other needs more because R. macrandra will get damaged near the light. Which of these CO2 need levels should be targeted as a general advice ?
Is there any limitation in the tank ? Less CO2 is ok then. Tom Barr even argues that low PO4 could help reduce plant problems in tanks that would be CO2 limited.
Like all the nutrients, C needs to be provided in non-limiting amounts for good plant growth. If you are unable to provide this level (no matter the reason) the best way forward is to reduce the light intensity. A large number of plants can survive and grow at lower light intensities. What exact ppm is non-limiting depends on your specific aquarium and point in time. For this reason I consider it good advice to recommend 30ppm for non-limiting CO2 levels in medium-high light setups.
Further reading: The light limiting growth management method - Aquarium Plants - Barr Report