Buy distilled water at the grocery store. Use it for flushing and mixing with fresh coolant. Avoid using tap water (or mineral water, etc). This REALLY reduces mineral deposits in your engine.

I have 3/4 of a container of BMW 50/50 on hand. I was trying to understand how much coolant would pour out when I disconnected the hoses to the t-stat. I'm thinking I can do this without a full drain but rather a top off/bleed after the t-stat replacement. The coolant in my X was flushed less than 2 years ago.

While I haven't done a forum search to see if your question has been answered in an earlier thread, I don't recall reading a thread that addresses your question. I suggest you try it and measure the amount needed to refill the cooling system. I would be interested in the answer.

Based on the location of the thermostat, my guess it won't be enough to matter. It would be easier and not make a mess to drain the system and capture the coolant if you want to use it again.

It's ok to reuse the coolant if you collect it into a clean container. Drain the radiator from the bottom drain plug which will get about a gallon out. When you remove the hose at the thermostat, have your catch pan or bottles ready as the engine/valley pan ( on a V8) will drain out there - another 1/2 to 3/4 gallon.

If I am reusing coolant (knowing that it was new, etc), I pour it through a large cone coffee filter placed in a large funnel to catch any scale or other junk that may have fallen in there. If it's really new as you say, it should be clear blue fluid you could inspect in a bucket.

+1.

Your coolant should be clean and suitable to be captured and used over again. Top as needed with the premix that you already have.

I haven't done this on an X, but on my E46, I did it standing in the driveway in January. I don't recall it being enough coolant to make a huge difference or even mess. Heck, I'm not even sure I removed the electric fan (XI manual), when I did it. I replaced the T-Stat, then bled it in through the overflow tank. Remember to respect the overflow tank and not fill it to the top, but to where the shoulder of the tank curves up to the lid.

I replaced the t-stat and did my belts while I was in there this afternoon. Quite a bit of coolant came out in the process. I'm guessing about 2 pints as a point of reference. Unfortunately I may have gotten some coolant and/or water I used to rinse the spilled coolant in electrical component(s). Car wouldn't turn over. Finally did after a few tries. Went in the house for about 20 minutes and gave it another shot. Same result. Got it started and took it for a 20 minute ride on the highway to assist in the drying process. Hopefully no issues tomorrow.

Getting something wet should not be a problem, else nobody could drive in the rain, or after leaving the car wash.

The main electrical item on the front of the engine, and near the job site, is the alternator, and it has no trouble being wet.

X started and ran perfectly today. Great sense of satisfaction on this DIY. I was quoted $593 for the T-Stat replacement by my local dealer. My Indy is tracking 30-40% less on average.

If you were working on your car and had doors open, or key on, or whatever, then you likely drained the battery if the No Start issue was that it was cranking slowly or not at all. If this is the case, you would do well to plug in your battery charger over night.