I agree with you, Kimball, that a 2,9º piece, to complete 15º together with the 12,1º of the 24912, would be very convenient. But we have to make do with the available pieces (please, correct if you find any mistake):
...................... 5,7º ... 7,5º ......12,1º ...... 15º ..... 24,3º..... 30º
R1 = 360mm .............. X .......................... X ..................... X
R2 = 437,5 mm .. X ..... X .......................... X ......... X ........ X
R3 = 515 mm ........................................... X ..................... X
R4 = 579,3 mm ................................................................. X
R5 = 643,6 mm ................................................................ X
R9 = 1.114,6 mm ....................... X
Two 24206 pieces make 11,4º, less than 12,1º of the 24912, so that dividing 24,3º (24224) into two is virtually exact if you consider the space lost while cutting it.
Seven 24912 pieces plus one 24206 mean 90,4º, the nearest approximation to 90º.
In any case these R9 curves, with their wide radius (over one metre) may make up for the lack of flexible C track, something like this:

Another possibilitiy is adapting real flexible track, as I show in my post of 17th February:
https://www.marklin-user...try-and-port-facilities. This allows a parallel track.
Regards
Paco