I think "Week number" is too ambiguous to be a guide. If January starts on the last day of the week, does that still count as week#1? What day does the week start on, anyway--much of the world starts the week on Sunday, much of the world starts the week on Monday.
"Fourth Tuesday in January/July" is unambiguous and makes everything easier to plan. Using "Start + N weeks" for the rest of the target dates is fine.
The whole table is meant to be a guide and not exact. I like having the approximate calendar dates, because I think this is the most informative for non-release managers. The week numbers are really meant for the release manager, but I would be fine replacing those with deltas (e.g. + 4 weeks) if you think that made more sense.
I'd expect the pace of preparing the release to be basically the same regardless of start date, so the milestones would make more sense as start date plus N weeks.
If you want wiggle room for the start date, that's fine, anything that expresses "late January" and "late July" will work. I thought it was going to be more definitive, what with specific week numbers and all.