I doubt there is any formula. However, a little "common sense" may help.
These statements from those articles can be used as a guideline:
Water chemistry: pH 6.8-8 (7.6), 8-30 dH (16), 72-81°F (22-27°C). A 2% addition of salt is necessary. This can be accomplished by adding 15 TSP. of salt/ 10 gallons (20 g of salt/10 L).
The water should have some sea salt added and measure a salinity of 1.005 to 1.015.
These fish need to be in brackish by the time they have reached several inches in length (3 or so). Since those articles do give a targeted level of salinity, I would simply divide the amount of salt it takes to produce that level and add it in several doses over a week or two as the fish reach that size. This assumes that the fish are not already suffering from not enough salinity. In that case I would be inclined to raise things much faster. It is very rare that moving a fish out of grossly wrong parameters into proper ones as quickly as possible will harm the fish more than not doing so.
Based on the idea of shooting for 15 level teaspoons of salt for each 10 gals. of water, consider adding 3 teaspoons/10 gals. every other day for 5 days. This lets you observe the fish for a while after each addition and if they seem to have a problem, you can pause the additions for a while.
Bear in mind that your tank does not hold its advertised volume. To calculate its actual volume of water, measure the height, length and width on the outside and then subtract twice the thickness of the glass panel from the length and width. For the height subtract one glass panel and then the amount of air space between the surface of the water and the top of the tank, You can then put those numbers into a tank volume calculator to determine your actual water volume. However, you will need to reduce this further to account for ant substrate, rock-work and wood (or other ornaments). I normally suggest this be 5% to 10% depending on how much of these one has in the tank.
To give you an idea of true volumes- a 10 gal. Aqueon tank without anything but water in it and allowing about 1/2 inch of airspace at the top would hold a bit over 9 gals. I have a 150 gal tank I calculated would hold about 135 gals. This is about 90% of the tank size it is sold as.