Another joist question...

So, I bought my pole yesterday \o/ I'm told it should be arriving Friday or sometime next week, depending on what the New Year's hours are for the delivery guys. I'm not going to be home tomorrow or Thursday so I figured I'd try looking for the beams today, get it all done before the pole arrives and I put it up in the wrong place in my excitement lol. I know this has been asked before and I just searched the forum for 'joist' to see what threads came up. Unfortunately I don't know if I'm even dimmer than I thought or my ceiling is made of cardboard 🤔 I knocked around and the whole thing sounds hollow 🙁 some of it sounds slightly lower and some of it sounds higher, but that doesn't make sense either - it's the air that makes the sound, so technically, where there's less air it should make a higher sound, right? Well in my room it makes the lower sound right where the light fitting is. And I heard someone saying the beams normally coincide with the light fittings... but if it's making a lower sound by my light, then the beam isn't there? So I went out and bought a stud finder. £25. Not millions, but definitely a lot considering I'm a student and I shouldn't really have bought the pole in the first place -_- and I can't take it back because of the packaging they use. Bah. I tried it, I think I figured out how it works, but it seems to just beep when it feels like it. Either that or my flat was designed by Picasso. Apparently you have to calibrate the stud finder properly, meaning, NOT on a stud/joist/beam - but um, how am I meant to know what isn't one if I don't know what is one?? I tried calibrating it on my wall like it said, but then it thought my whole wall was made of stud and my ceiling was dotted with little ones. And in defeat, I came here to see what others were saying...then discovered that stud finders normally don't work, and that they especially don't work on plaster ceilings, which I think is what I have. There goes a new pair of shoes I could have bought with that money T_T I'm going to try it again in a second to see if I can figure it out, but right now I have random lines all over my ceiling that are meant to be stud edges but really can't be... I'm in the UK so I don't have the 16" distancing. Anyone know what the distance is round here? I'm also tempted to do the nail thing; I have one problem with that - I don't have any nails, and I don't want to completely destroy the ceiling. And I realise that was two problems. My flat's rented, and basically, if I ruin a little bit of my ceiling (did I mention it's Artex...?) that's okay - either they won't kill me (there were bits of rough carpet and stuff when we moved in so maybe I'll pretend I didn't notice) or I can fix it myself (probably with acrylic paint so it just looks the same. Hehehe) or I can just pay for that little bit to be redone - I've got ages til I move out so I'll have time to save. A few tiny little holes I can definitely fix up myself... ...Unless it makes the whole ceiling cave in. I've got a whole 'nother flat above mine, a roof, and probably a loft, too. If it caves, I'm dead, so are my cats, my mum, and everyone upstairs. There's actually already a crack in the ceiling, that runs a little way down the wall towards the edge of my doorframe. Only noticed that just now =| So uh yeah in conclusion (I never did learn how to make short posts on a forum) how the hell do I find them!? How much damage would it do if I put some nails through the ceiling? How many would I need? And, perhaps a stupid question, but d'you think I could do it with sewing pins...? They'd make much smaller holes...