Frequency is a metric typically expressed as "it happens x amount per second", interval is "there's x seconds between each time it happens". They're basically the same thing, just a different way of showing.
You're thinking of the +3 breakpoint for glyphed renew. A normal Glyphed renew means 9s base duration, 3s base inteval. at 0% haste that's 3 ticks. +3 ticks at 83.32% haste means 6 ticks total.
I was talking about a refreshed glyphed renew. Refreshing it through the serenity mechanic gives you a base duration of 12s with a base interval of 3s, that means the breakpoints of the unglyphed renew apply.
That's +3 ticks at 62.47% haste, for a total of 7 ticks.
If you can achieve this, it's a ridiculous amount of healing per renew, although I don't think it's worth it. Go for 13k breakpoint, lots of spirit and always use the glyph. The 18k breakpoint might not be worth going for if you're using the chakra mechanic to it's maximum potential