a 1/8" slot, or 3.175 mm is what is generally recommended if you are using the #7 Mesh Plastic Canvas, which is about 1mm thick and leaves 1mm of slot on either side.
I think your problem however is the uneven cut of the slot. It is key to even flow is to ensure that the slot is straight and even width. If it's not straight, the screen can get flow blocked to areas. If it's not even width, then the flow will vary across the screen. Also key is cutting the slot such that the screen can hang perfectly vertical, which means the cutter must me at a perpendicular to the tube while cutting if this makes sense. The screen needs to basically hang freely, only held in place by the zip-ties.
You can make the slot a little wider, I think mine is probably close to 4 or 4.5mm and I currently run less than ideal flow across it, 22" slot with a Mag 5, and the flow doesn't start out completely even after cleaning, but one day later it's perfectly even.
Bottom line here is that you need to cut another slot tube. Take your time and do it right, mark the slot and cut it straight. Start with 3 or 3.25mm, hook it up, check the flow, widen in small increments as necessary.
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