Basis on a dated future

A future is $81,200 and spot is $80,000, with 91 days to expiry. Which quantity do you annualise to compare it with funding?

• The future's price change over the 91 days
• F ÷ S − 1, scaled by 365 ÷ 91
• The spot price's yearly return

When is a dated-future basis actually locked in?

• On the day you open both legs
• Whenever the basis is narrower than at entry
• At expiry, if both legs are still open