QTR-Quadtone RIP

I've recently started to make some curves for my 2100 using a PrintFix
Pro. I've only used a densitometer previously so I'm not that used to
dealing with AB values in addition to L. 

I'm using a setup with 1 black, two grays (LK and LLK) and LC and LM
color inks for toning. 

I can manage to get within +-1.5 AB accuracy across the entire curve,
but beyond that is starts to become difficult to improve accuracy. 

Here's my current linearized curve for Permajet Alpha:

1	97,78	1,36	0,38
2	93,45	1,12	0,06
3	89,36	0,99	0,12
4	85,52	0,92	0,26
5	81,54	0,94	0,52
6	77,12	0,95	0,81
7	73,33	0,88	0,85
8	69,08	0,92	0,89
9	64,86	0,93	0,88
10	60,91	0,83	0,63
11	56,66	0,71	0,52
12	52,41	0,74	0,49
13	48,19	0,79	0,52
14	44,27	0,91	0,60
15	40,30	0,97	0,74
16	36,09	1,03	0,62
17	31,61	1,07	0,37
18	27,93	1,17	0,41
19	23,43	1,21	0,58
20	19,93	1,18	0,60
21	16,26	0,89	-0,03

To be honest it's pretty hard to visually tell the difference between
the last 10 stepwedges I've printed where I tried to improve the
accuracy within the 1.5 range, so I'm wondering if this is good enough.