Sounds to me like you’re getting it down. Whenever I’m trying to emulate some classic device, I tend to study the original parameter documentation (if I can find it), then fine-tune by ear. Break the ‘rules’, or expand on the parameter ranges, as needed.
Yes, the manual / delay time concept is counterintuitive: shorter delay times = higher frequencies. You can get flanging with any delay algo that’s short enough, and has modulation capability (ex. Mod Delay). One reason the Rose modulation sounds so damn good – fractional delay times, down to microseconds. Each manufacturer / design team is going to approach that fact a little differently.
One little tidbit about the O.G. Flanger algorithm. You may be jumping on Negative first, but in Thru-0 mode, the left output is positive flanging, with the right being negative flanging. With the Instant Flanger, you may also want to explore the negative Feedback option. I think the documentation for this vintage rack effect is done very well, but it doesn’t mention that nugget of info:
https://cdn.eventideaudio.com/manuals/h90/1.5.2/content/algorithms/modulation.html#instant-flanger
