You're fine up to ~1.4v Vtt, some say it's fine to go up to 1.45v, although I'm hesitant to recommend that. Your GTL's are fine so if you keep Vref under about 1.0v. That means GTL Ref x Vtt = <1.0v. If you have 1.2375v Vtt and you are using 63% GTL Ref (0.63x) your Vref is 0.779v which is absolutely safe. If you use 67% you get 0.829v which is still very safe.
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