It sounds about right for GH6 (6 drops is roughly 120 TDS). Those GH test kits are not the most accurate. And RO water reading of 3 is not unreasonable.
I don't know why you would have been reading 240TDS last week and now it's a 100 points lower.
I would suggest, not changing water parameter with a water change for a week, then see where it is at. Change if it gets close to 200 (say around 180-190). Let's give the tank some stability by not changing anything for a while.
I checked my TDS meter and it reads 3 on RO water and today my TDS in my tank is 133. I did a test with API and it took 6 drops so that's around 100 TDS.
I had 2 adolescents die overnight.
Should I only change water when it's over 200 TDS or should i continue with the 10% every week?