I didn't have a protractor when I flew either, but lets look at the available data:

In Executive/Premium Economy:
From Seatguru:
-- Singapore: Pitch 37" (width 20")
-- Thai: Pitch 42" (width 18")
From OP:
-- Singapore: 8" recline (horizontal measurement)
-- Thai: 135 degree recline

Let's assume (here we go with assumptions):
1. That both airlines wants a similar amount of space remaining at head level for a pax seated fully upright if the pax in front fully reclines. In other words, since the TG pitch is 5" more than SQ, lets assume that the recline is also 5" more at TG, i.e. 8"+5"=13" of recline measured at top of seat back.
2. The the height of the seat back is 30" (I have no idea how correct this is, but it's probably not way off)


So, with a little math (for those that didn't sleep through geometry )
The results:
@:-) 1. The TG seat (and the SQ seat by above assumption) in fully upright position has a recline of 105 degrees.
@:-) 2. 8" of recline will bring the SQ seat to 122 degrees recline.


Some comments:
- Yes I'm having a sloooow day at the office....
- My assumption 1 above leaves a "resdiual pitch" at head level of 37"-8"=29" in SQ EE if your seat is fully upright and the pax in front of you fully reclines. As a reference point, CO lists their 772 with 31" pitch and 5" recline, leaving 26" of "residual pitch" ... no wonder it feels cramped
- I'm really not sure if this makes the OP mcuh wiser (122 vs. 135 degrees), what does that really tell you?
- And finally, Kiwi Flyer, if the SQ seat back was 8" high and reclined 8" it would be a 180 degree recline