Most players use the thumb on their dominant hand (right thumb if right handed, left thumb if left handed). There are a few people who play alternating thumbs, but it's pretty rare.That makes sense, thank you! I'll use that in the future now!
Also, instead of referring to keys by whatever key you're using, refer to them by their column number. For example, f+space would be 34, and space+j would be 45.
Using this system:
S = 1, D = 2, F = 3, Space = 4, J = 5, K = 6, L = 7
If you want to refer to a pattern rather than a single chord such as S+D then D+F after, that would be 12/23, using the slash to separate the chords.