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depends on the dive profile, what tanks you're using, and what tanks your buddy was using, this is why gas matching is important.
Situations
You are a small female with a SAC around 0.4 cubic feet/minute.
Your buddy is a large out of shape male with a SAC ~1.0cfm
*easy math here so go with me
You dive an AL60 with a service pressure of 3000psi
He is diving a 120cf tank with a service pressure of 3000psi
He runs out of air, you have 750psi left, i.e. 15cf.
He uses 1.0cfm at surface, you use .4cfm at surface, for total of 1.4cfm at surface. You're at 66 feet, come up to your safety stop and he runs you out of air because at 66 feet your cfm is now 4.2 and at your safety stop it's 2.1. it takes 2 minutes to get to the safety stop depth, so call it 7cf used there, and 3 minute safety stop is another 6. You're cutting it real damn close. Depending on your body time, which is why you have to consult tables, a 3 minute safety stop with the risk of you running out of air and not having anything left in the tank is far less "Safe" than making a 1 minute safety stop and finishing your ascent. If you were pushing your NDL's, then it would make sense to get as much of that safety stop in as possible and then try to finish what what you can and hop on O2 if possible.
The 500psi return to boat should really be the CF required for you and your buddy to make a safe ascent from depth to the surface with safety stop, and that has to be recalculated every dive.
Situations
You are a small female with a SAC around 0.4 cubic feet/minute.
Your buddy is a large out of shape male with a SAC ~1.0cfm
*easy math here so go with me
You dive an AL60 with a service pressure of 3000psi
He is diving a 120cf tank with a service pressure of 3000psi
He runs out of air, you have 750psi left, i.e. 15cf.
He uses 1.0cfm at surface, you use .4cfm at surface, for total of 1.4cfm at surface. You're at 66 feet, come up to your safety stop and he runs you out of air because at 66 feet your cfm is now 4.2 and at your safety stop it's 2.1. it takes 2 minutes to get to the safety stop depth, so call it 7cf used there, and 3 minute safety stop is another 6. You're cutting it real damn close. Depending on your body time, which is why you have to consult tables, a 3 minute safety stop with the risk of you running out of air and not having anything left in the tank is far less "Safe" than making a 1 minute safety stop and finishing your ascent. If you were pushing your NDL's, then it would make sense to get as much of that safety stop in as possible and then try to finish what what you can and hop on O2 if possible.
The 500psi return to boat should really be the CF required for you and your buddy to make a safe ascent from depth to the surface with safety stop, and that has to be recalculated every dive.