Before any calculator, run the one-digit version: round every value to a single digit, multiply or divide in your head, and count the zeros. $3.9$ of $5.2$ grams? Call it 4 of 5 — about three-quarters. $4\ \mathrm{mol}$ at $300\ \mathrm{kJ/mol}$? Four threes and two zeros — a thousand-ish kJ. Five seconds, no buttons.

Here is why it pays: a slipped decimal SURVIVES a calculator — the screen shows confident garbage, ten times too big, and nothing on it blushes. It never survives an estimate, because a wrong power of ten stands a full 10× from your anchor. Estimate first, compute second, and let the two shake hands before you write anything down. Professional scientists never stop doing this; they just stop admitting it.