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.
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.