Yes, you eyeball it vs. the tape tension rollers position.
The basic positional geometry looks fine, but one word of caution that may be relevant to your problem.
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In the 388 manual the table denoting reel/
servo adjustment pots,.... RW Tension (R206) and FF Tension (r105)are BACKWARD. A MISPRINT.
So, when you think you're adjusting FF tension, you're actually adjusting RW Tension, and vice versa.
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This took me hours of T&E to determine. I even pencilled in the correct pot #'s in one of my manuals, but apparently not every copy. If you're adjusting FF when you think it's RW, and RW when you think it's FF, you'll basically never get it right. Take that from experience.
I think from the looks of your video you have things set up ok in a basic sense, and your problem can be adjusted out with more effort.
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SO, CORRECT ME IF I'M WRONG, BUT
FF Tension = R206
RW Tension = R105
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I'm going off memory, guys, but it's a pretty vivid memory when I discovered this pertinent misprint. Tascam manuals aren't prone to misprints, but this one got out there. If I could find my second copy of the manual, would show where I pencilled in the corrections. For now, just going off memory and intuition. Ok.