You probably need to skip #$10 every #$10 bytes (:= 2 letters). Or better, rewrite the routine that does the tilemap building. According to the other thread, one 16x16 character was built like this

`0 1`

2 3

where 0...3 represents the order of the 8x8 tiles in VRAM. So, when you only increase by #$08 (:= one 8x8 tile), you get to position 0 (your A), then 1 (your B), then 2 (:= next line; your C)! If you wanted to do a nice 8x16 font, you'd have to either a) interleave top and bottom (render top to 0, then bottom to 2, top to 1, bottom to 3, and so on) or b) rewrite the tilemap such that it's

`0 2`

1 3

Well, that's how I *think* it works, judging from this and the other thread.