As with all new CR-Z owners you are over thinking the charging of the IMA battery.
The best advice I can give you is just ignore it most of the time and you will find it takes care of itself. It will recharge as you are coasting and driving at a reasonable speed without braking.
The rate of charge is variable based on what mode you are in and how you are driving. Riding the brakes, even a little will wear them quicker and heat them up and possibly cause damage.
Think about defensive driving and learning as much about how the car handles and drives rather than the fact it happens to have Hybrid technology. I found once I started just driving, the charge stayed higher than when I was worrying about it. I find on long parkway runs I get a little more charge ( and less MPGs) if I use sport. The first year I owned the car I left it in Econ because I was all about the MPGs. Now I use different modes based on what kind of driving I am doing or the situation. My experience will be different than yours as I have a CVT.
Or to summarize just drive it!
I hear you! If I wasn't ever so slightly obsessive, I would have chosen a different car...
The real revelation for me last night was there is a balance between MPG and keeping the battery up. I found I was able to keep more charge by accelerating robustly in sport which opened more opportunities to regen. I'm not a commuter, as I work from home, and gas is cheap, so I'm not all about the MPGs.....I honestly bought a fun two seater hatchback stickshift, not a hybrid. Keeping the battery up is important because it's a bit of a dog when it's depleted but a hoot to drive otherwise.
When I say riding the brakes, I hear you on wear and heat, but I mean ever so slightly touching the brake.....max regen still means driving like a jerk, because it means riding the brake ever so slightly, but the faster you're going, the more regen you get......so, long distances touching the brake then coming to relatively abrupt stops at the end to maximize the time you're in regen at higher speeds. It shouldn't be necessary most of the time though. Still, no doubt that's max regen, it's the only time the regen bar is maxed, and it's totally driving like a jerk.
I would prefer full regen when downshifting in econ mode, but I won't get it. Braking then getting off seems to split the difference, so cool, and when it's really low go for the power regen described above. It is tempting to just leave it in sport and let the regen meter and MPG be my guide. The recommended shift points are not only boring, but they definitely do not promote regen. I know this because I was driving to max regen, but the car was angry with me, showing blue on the speedometer and illuminating the upshift light. I shifted, and the regen went down. Driving reasonably in sport, by not going into the burning part of the meter all that often, would probably result in acceptable MPGs.....33 is good enough for me.....and the red dashboard and lack of shift point nannying is just fine.
I'm delighted with the car. I mean, I was driving a '94 Saturn with 197,000 miles on it that rattled everywhere, so any new car would be great haha but this thing is fun!