Hey, welcome to the forum! I love linden tea, I bet linden honey's awesome.
10 litres is about 3 US gallons, last time I had to fix a 5 gal batch I was adding calcium carbonate by the teaspoon, so I think if you added half a teaspoon of your mix of carbonates and tartrate at a time, stir it well and give it a day or two to balance out, you shouldn't overshoot and end up with something chalky-tasting. And I'd aim for around 3.4 or so.
Just be sure to degas it first, not only will the dry powder provide plenty of nucleation sites for your mead to fizz up but also (if you recall science class where you mix vinegar and sodium bicarbonate) the chemical reaction of adding base to your acidic must could fizz up, so even if you mixed your powder into water to dissolve it before adding, carbon dioxide that's already trapped in the must could end up helping the fizz from the chemical reaction get out of hand.
"The main ingredient needed is 'time' followed closely by 'patience'." - The Bishop 2013
"When you consider that laziness and procrastination are the fundamentals of great mead, it is a miracle that the mazer cup happens." Medsen Fey, 2014
"Sure it can be done. I've never heard of it, but I do things I've never heard if all the time. That is the beauty of being a brewer!" - Loveofrose, 2014
"I tend to....um, er, experiment, and go outside the box. Sometimes outside the whole department store." - Ebonhawk, 2014