Buy the amount you'll actually get
The pre-fee number is a lie you tell yourself. Size from the after-fee coin count and know your break-even from the start — see the break-even calculator.
Working backwards from the target
Most people decide how much to buy by looking at their available balance. This calculator flips it: start from what you want to end up with, and it tells you the position that gets you there.
Example: you want $500 profit if the coin runs from $2.00 to $2.50. That's a 25% move, so you need $2,000 in the position — 1,000 coins. Suddenly the question isn't "how much can I buy" but "am I comfortable putting $2,000 on this idea?" Different question, better decision.
The classic mistake is anchoring on coin count instead of dollar exposure. Owning 10,000 of a $0.20 coin feels bigger than 0.02 BTC, but at $2,000 both positions gain and lose identical dollars per percent move. Price-per-coin is psychology; dollar exposure is the position.