16 delta and 21 DTE

The stock is at 200, IV is 20% and the expiry is 45 days away. Roughly where would 16-delta short strikes sit?

• About 196 and 204
• About 186 and 214
• About 160 and 240

Why does tastylive's time guideline land near 21 days for a trade opened at 45 days?

• Because options lose all their extrinsic value at 21 days
• Because short gamma risk starts rising around the option's half-life, and their study's biggest losers came after that point
• Because exchanges change margin rules at 21 days