It's not necessary to structure cycles around exact 1 mL values at a time, but it's convenient. The same is true for whether administration needs to be per week, or different.
Here, I'd suggest 250 mg per 3 days, which works out to an average of 583 mg/week. Ordinarily that's a little lower than I'd usually recommend but you did well with only 400 mg, so it's unlikely you won't do well now with this amount. You appear to be a good responder.
However I'd inject 3 mL on Day 1 as a frontload (it's a slight overshoot but not where it will be a problem.)
If you have 21 ampules total, then that leaves 18, which will make it an 8 week cycle approximately, counting time suppressed. Works out very well.
However, if you're particularly curious and in a rush on that curiousity to see how you do at 750 mg/week, that would be fine too.
In either case, if you didn't use an aromatase inhibitor in the first cycle and got away with that, you might not be equally lucky with the higher dose cycles, particularly the 750 mg/week cycle.