I thought I'd try a new strategy and missed the 50-50 shot at 3, so my last two guesses were the same as yours. A different 2nd guess might have narrowed it down to 1.
Here's a little command line crunching of the 17,723 possibilities:
[char] [total matches] [col.1 matches] [col 2 matches].....[col 8 matches]
= 17723 0 0 0 0 1318 10515 5890 0
1 10768 3255 1133 1048 2017 1695 1063 2525 1698
2 9274 2107 1292 793 1949 1349 902 1510 1868
4 8368 1846 1240 789 1547 1251 783 1186 1839
3 8225 1875 1069 647 1645 1281 787 1245 1648
6 7992 1733 1194 802 1140 1192 722 1055 1844
- 7810 0 649 4829 1323 1699 0 0 0
5 7678 1716 1138 730 1321 1208 726 1095 1740
8 7588 1708 1159 844 743 1152 695 976 1881
7 7274 1683 1011 674 860 1159 655 1003 1657
9 7198 1800 964 776 537 1185 646 1024 1718
+ 6839 0 2002 3405 1606 471 0 0 0
* 5574 0 2978 774 1256 940 0 0 0
0 4464 0 1480 258 252 720 229 214 1830
/ 4024 0 414 1354 1527 1103 0 0 0
Note: - is used more than + , = is most likely in column 6, and column 1 is a 1 18% of the time.