Sounds like you need to find an instructor that will do a non cert workshop focusing on buoyancy and trim tailored to you specifically. I do this for people all the time. I once had a guy come in and we spent 4 hours in the water where we got him trimmed and properly weighted using his 3 mil suit, 5 mil suit, and 7 mil suits. Then we wrote down all the various weights that were needed and where they were placed. We used al80s because that was what he usually rented.
I've also done it with people using steels.
You don't hold the breath to see where you float. You weight yourself so that you can exhale and descend and with a cylinder at 500 PSI you should be able to hold your safety stop.
If you can't descend with an empty BC upon exhaling, you don't have enough lead.
"Edit: by "slight trouble" I meant I usually had to bring a rock or two
then throw them away once I reach the bottom (~15-20') ..." This is a good way to end up in a bad situation when you don't have enough weight to hold a stop.
What kind of BC are you using? Does it have trim pockets? Many do but they are placed so they are mostly useless.
Are you using a weight belt in combination with the BC if it's weight integrated?
How do you have your weight distributed?
How many times did your instructor make you do a check?
I have an entire chapter in my book on weighting and trim and I tried to post an excerpt but it won't let me copy and paste.