I bought an older house that was built in 1971. It is a 2 story home. 1st floor is completely finished half basement that has 4 rooms. The utility room, which is directly under the master bedroom has the plumbing and joists exposed. The rest of the rooms has a finished ceiling. The 2nd floor which is the main living area, is all hardwood flooring. I believe it is the original flooring installed in 1971. It's not in bad condition, but EVERYWHERE you walk and in all rooms, the floors squeak. I went to Lowes and bought this "screw down" system where you screw these specialized screws through the floors into the joists and it pulls the hardwood down tight to the joists and stops the squeaks. It really works well. My problem is that I can't find all of the joists on the 2nd floor. The only one I can find are the ones in my bed room where the joists are exposed below and I can kinda map it out from below and then go into my bed room at one of the common walls and do my measurements. I can't do this for the other rooms as the ceilings in the rooms below are finished. Another issue is I discovered that in the utility room, the joists aren't the standard 16 inch separation between them. Some joists are 12 inches apart, other are 11, some are 13. It varies. So, I can only imagine it is like this throughout the whole house. So my question is how do I accurately find the joists through my wood floors on my second floor so I can screw them down when finding them from below is not an option? I have purchased a high end, deep scanning stud finder (Precision Sensors ProFinder 5000+ Deep Scanning Stud Finder) and it is inconsistent at best. I am at a loss and not sure on how to resolve my squeaky floor issue. Any ideas from the Pro's on this site? Please help!

Since the sheet rock below is fastened with either nails or screws, you can determine the layout with a stud finder or a magnetic stud finder and then transfer measurements to the overhead room. It will take patience to get the whole layout.

The first floor ceiling sheet rock should be attached to strapping and not directly to the floor joists so sheet rock nail or screw locations will locate the strapping and not necessarily the floor joists. Can you tell if there is a subfloor under the hardwood or is the hardwood sitting directly on the joists? A good deep scanner may go through 1 layer of 3/4" wood.
Another option would be to get an inexpensive inspection camera, make some small holes in the ceiling sheet rock, and use the camera to locate the joists....will have some small hole patching to do but you'll be able to definitely locate the joists.

If you are determined to find the joists, from the room you are in, you can use a drill to find the joists. You already have the right color putty to fill the holes from the screws you will be fastening the floor with. Just use a one eighth inch drill and drill every inch along both walls then snap a chalk line or stretch a string between them when you find solid joists under the floor.