I don’t know those shifters so excuse my ignorance if there is a complicating factor. But it’s most common just to have a shifter pull/release a set amount of cable. So you normally do all the adjustment at the derailleur.
When you say you can’t get it right is that something like working for the first few gears and then not shifting immediately in later gears?
Have you changed anything with the cassette or rear derailleur?