Herbie drain - Fluctuating overflow level

[Potatohead]

This is my second tank with a Herbie, they work well but I am finding I get about 3 - 4 cm of fluctuation in height in my overflow box. My old tank wasn't super stable but probably better than this one so far. I can adjust and it will stabilize, go back in twelve hours and it has hardly moved, but then another twelve hours later it has moved a couple centimetres or something. My plumbing is as simple as it gets, I just have a straight drain to a 90 and then soft tubing in a soft bend to the filter sock. I literally have my gate valve sitting vertically on the filter sock mount. I am using an ATO that allows about 1.5 cm return chamber water level difference from high to low.

This tank has been running about three weeks. Will this possibly become more stable in time? Should I run a trickle down the emergency? Should I run a ball valve on the return and throttle the pump (Tunze 1073.02) back just a touch? Is this kind of normal since filter socks don't always allow flow through at the same rate and/or because of atmospheric pressure?

Thanks