There are different approaches and you can use any method really, depending on what you want to achieve?
Initially you are wanting to reduce the overall TDS, but not too drastically so I would do 10% (1.5L) water changes with new water of about 100TDS, ona SLOW drip and that will take you to 275 from 295 first change and take it from there. The TDS will increase over a period from evaporation unless you top it up with zerowater between changes. At this stage you then have to decide upon how often you do a water change and to start with I would try 2 per week at 1.5L of TDS 100 slow dripped. After the second change you should be about 255 so virtually there so I would then go to NORMAL maintenance regime (next paragraph). INCIDENTLY YOU WILL GET THE EXACT SAME FIGURES IF YOU DO THE SAME CHANGES AS THIS, BUT JUST 1L CHANGES WITH JUST ZEROWATER, which is what I think your question is so it is up to you which you do? As you have some nitrate readings I would probably do the 2 x 1.5L TDS 100s personally. Do slower dripper though than you will normally as you are ending with 20TDS change in water!
I am assuming 2L once per week, when the spreadsheet will be more useful for better accuracy. First 2L waterchange of TDS200 will take you to 248, and as long as you keep topping up with zerowater in between changes from then on you change with about TDS250 new water, so you should be all set from then anyway! As the old and new waters are virtually the same you can do faster dripping than when doing bigger TDS changes.
This is of coarse based on variables such as 'have you kept the tank topped up', sometimes I get lazy nearer water change time so end up taking out less than I add new water but that can be worked out using the spreadsheet to work out what you need to mix the new water too, to get it right, hence tomorrow I will be removing 2L of TDS 165 water and putting in 3L of TDS 140 new water to get it back to TDS160 and at the right fill level for the tank.
I think I would do as you are and leave the duck weed in this tank as it is probably absorbing some nitrates as you say so forming part of the cycle. I wouldn't use it in the new tank at all though, just use the water lettuce etc which is much easier to deal with.
Hope this helps and you should be running as a normal parameter tank with a regular regime to make life easier and straightforward in a week!
Simon