I would basically ignore it. Domestic recipes are generally never that accurate anyway. Does the recipe mention level or heaped tablespoons? Does it say 20 ml for a tablespoon?
For rising agents you may want to be a little more accurate, so how about 1 tablespoon and one teaspoon, or just 4 teaspoons, or even just heap it a little?
I have a few Australian published recipes with dual measurements, and they show tablespoon as 15 ml, so the whole 20 ml thing may not be so universal in Australia in more recent times. Some examples are:
Many Australians are recent immigrants, including 500,000+ from New Zealand and they all use 15 ml tablespoons
Another issue being that now all the basic kitchen utensils are made in China, so they get what every one else gets, 15 ml
In the history of the tablespoon it has meant many different sizes in many different cultures, from South Africa with 12.5 ml, to India with 25 ml