I just alternate on topups. I'll use straight G12 once and then water the next time. I keep both in my trunk. Unless you were filling up from dead empty, filling the reservoir with some straight G12 isn't a big issue. Draining isn't really necessary, IMO.
Also, I think, depending on where you live, weather, etc. You can run up to a 70/30 mix coolant/water. I really wouldn't worry about it.
Mine: 20th AE GTI, BMP, 133k, 3" Turbo back, Stage 2 Unitronic
His: '16 GLI SE 6MT
The project: '84 Jetta GL TD and '82 Caddy ABA