How far you can drain is really dependent on your water table level. So you need to be cautious but 25% of pool water seems good but you may want to talk to any neighbors that have pools to determine if they had any issues.
I calculated my depth per inch (rough estimate) using my surface area of pool. So I would then estimate that my first step (at 6 inch water depth) was x gallons, my second step at 18inches water depth was x gallons. Obviously every pool is shaped differently and how it slopes all affect calculation but at least you are relatively close. So lets say you drain to bottom of your bottom step. When you start to refill, take a picture of your water meter reading as the starting point. Then when you reach the top of the bottom step take another reading of your water meter and so on. This way you will know at the end what you removed/added and also for the future you can then have a better calculation what volume is at each depth of your steps
I removed all my return eye outlets in both pool and drain - removed any debris or build up and set up to properly direct flow. You be amazed how people may play with these things and they get all pointed differently.
Chlorine water is not good for grass or plants - suggest you drain to street or to some type of drain
Depending on how your plumbing is setup, you may drain the spa with the pool. I have drained my spa separately using the bottom spa drain to remove all water. You can calculate the volume of your spa much easier - just use 2 rectangles (one with max depth in middle) and other with depth of seat.
You may want to look at your water bill. If your water usage also affects the fees for garbage services or drainage fees, then call the water company to tell them you are filling a pool so you do not get those extra fees on your pool volume.
Good Luck. Have you tested your fill water so you know what you are filling it with?
-spillover setup.
