I found a way to find songs and their PROPER respective BPMs exactly on the dot.
Like I said in the title I found a way to absolutely find the exact bpm for the music you like. I don't know if some players found this already but might as well share this for those who don't. Just search [Music title] OSU BPM. It gives the (not all of em, but most of them)precise bpm on the dot which OSU players mapped out already for certain songs. You still need to adjust the offset timing tho.
Here are my songs that I got everything on the dot (like no actual delays/beat going first, again you need to adjust the offset yourselves) since I searched (Music title) OSU BPM:
Godzilla - Eminem Ft. Juice Wrld = 160 BPM, Offset 1158, Start 1.462
Don't Stop the music - Rihanna = 122.71 BPM, Offset 264, Start 0
Bye Bye Bye - NSYNC = 172.48 BPM, Offset 553, Start 9.076 (tweaked it a bit for myself but still almost the same as the OSU BPM one)
Aizo - King Gnu = 190 BPM, Offset 530, Start 3.07
Take on me - A-ha = 169.01 BPM, Offset 297, Start 0 (okay this one I just did a little bit of tweaking since OSU and Dead as Disco plays relatively differently anyways still close to the OSU BPM, it made it easier to find the BPM since it has a period on it.)
Smooth Criminal - Michael Jackson = 118.21 BPM, Offset 61, Start 8
That's the only list I had for now since I only recently had the braincell to search for the [music title] OSU BPM xd. I got burnt out from mapping since I spent an hour and a half mapping Bye bye bye (I know it's way too long XD) before I realized the search thingy since Dead as disco is my first proper entry to rhythm games.