On a 960, the low pressure charge port should be on the dryer (the big can looking thing by the firewall).
People will argue about how best to service A/C until the cows come home.
I'd postulate a few obvious principles (OPs) that might make it easier to decide:
1. Your car isn't brand new, or even say 1-3 years old where it might just need a top-off...it is at least 12-18 years old being a 960. Service might be as straight forward as a 1-3 year old car, and that is great if it is, I'd be happy, but most likely it isn't.
2. Whoever worked on your car in the last 12+ years might have charged it with something different than OEM or sealed it or not differently than OEM.
3. There may have been periods of time in that last 12 years where contaminants or moisture made it into the system either from a part failure (compressor or some such) or from disuse (moisture is common)or a puncture/major leak.
I've seen some PAG oils chemically make a mess of A/C systems used in volvos, seen rusted up or chewed up compressors and seen the dryer have a bunch of oil sitting in it.
I would make sure everything seals well (new o-rings of a nice material go a long way, but are time consuming to install), check for any debris from the compressor, change the dryer (they are cheap and recommended) and vac it down, verify it holds vac and charge it up (you may have to jump the compressor low pressure switch to kick it on for a sec).
Lots of oil can wind up in the dryer. I charge by weight, but verify by the gauge too. There can still be a lot of oil in there even after a vacuum, particularly if the compressor or dryer aren't replaced which takes up volume and can lead to over-charging if one puts the proper weight in only. Of course, all this extra oil, though not real harmful, decreases the cooling (heat exchange really) ability of the system, so I check that both the weights, pressures and vent temps are close.
This may sound like a lot of work, especially if your compressor and drier are recently replaced or known to work. Simply a vac and charge or top off may be "good enough" to make it work and keep working as long as you keep the car. If it has some refrigerant and will cycle rapidly, I'd add a little dye and top it and forget it and sell the car in a while...its a 12+ year old crappy volvo lol.
I guess the final corollary OP is that if you want your 12+ year old A/C to work like brand new, you are going to have to spend some real time and be willing to change or significantly inspect all the parts (doing things like looking at each piece and verifying all oil is drained and everything is dried). Making it work OK when it sorta works already shouldn't be impossible though either.