How do I make line calls when there is no official?

Tennis Canada's Guidelines for Unofficiated Matches (Rules of the Court 2026) say:

• You make the calls for all shots landing on, or aimed at, your end of the net.
• If you have any doubt, the decision must go in your opponent's favour.
• If any part of the ball touches the line, it is good: "A ball 99% out is still 100% good." Do not call a ball out until it has landed outside the court and you can clearly see a space between the ball and the line.
• A ball you cannot call out with certainty is good. You cannot claim a let or replay because you did not see it.
• Your opponent is entitled to a prompt audible and/or visible "out" call.
• Except for the first serve, call against yourself any ball you clearly see out, even if not asked.
• Treat every point the same, match point included.
• Serving: neither you nor your partner should call a first serve out, but call any second serve you clearly see out.
• Doubles: either partner may call a ball they clearly see. If you and your partner disagree on whether your opponents' ball was out, call it good.
• On clay, if any part of the ball mark touches the line, call it good.