Nothing wrong with trying to determine whether one sinned or not. The Pope, Priests, apologists and others provide advice on the matter as it relates to Scripture. The example you provide above about “spilling seed outside the vagina” is addressed in Genesis as it describes the sin of Onan. Surely you’ve read that before!? Search CAF on the matter, there’s plenty of threads on it as you obviously can imagine. The discussion of the sin of Onan was very specific regarding the unnatural selfish act he committed and how God detested it. He did it to deny creating life and for his own pleasure after he was told to marry his brother’s wife first, so he could then create offspring.
The 10 Commandments are somewhat broad and all encompassing. For example, Jesus clarifies this when describing that if you lust after a woman you’ve committed adultery in your heart, right? So its the same thing. As with the 7 “deadly” or capital sins! And btw, the Methodist Church does acknowledge the 7 capital sins.

I suppose its whether you wish to pick and choose which ones you THINK are related to them. We simply need to advice from time to time to help determine whether some act we committed (or omitted) is a sin.
When you do an examination of conscience, how do you determine what sins you’ve committed (or omitted)? Scripture like the rest of us? Pastoral advice? If you are left wondering and can’t seem to locate it in Scripture, how do you know? If you can’t find it, do you just shrug it off as though it doesn’t exist so whatever it was must be ok?