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Say, I invest $1000 in a stock valued at about $10/shr last June, in 2025 (so, 100 shares). Since then, I will have invested a few more times into that same stock in smaller amounts. Today, the stock blew up to $100/shr, and my original investment is worth $10K, so I go to sell that original 100-share lot (keeping in mind, all shares owned are still short-term at this point) while keeping the rest of the later investments in it.
If I wanted to pay tax on the profits of that sale only right away (+$9K, without concern over short-term rate vs. long-term), just to ensure my normal tax work just before April of next year doesn't look crazy with an additional potential $2250 or more owed (assuming the short-term capital gains tax is 25% and that if left, it might earn interest), then...
1) Is it really a good idea to pay it right away, or should I wait and just reserve approximately that much anyway (and maybe hope to gain interest on it, too)? How does that potential interest play into that, then?
2) Is it estimated if you do go to pay it "early"? How is it calculated? Does the broker provide that accurate info immediately? Can you overpay without issue to play it safe if you don't have accurate calculations yet?
2a) Is it (or can it be) adjusted to be accurate later (assuming it was close, but wasn't $2250 even, based on the actual amount you sold at, or, the tax rate isn't an even 25% for whatever reasons) while doing your taxes in 2027 (for 2026)?
How does that work, exactly? Any advice for this or if/when you catch a stock that effectively "moons" and gives back large gains? (Or just for any gain, really)