Simply take negative indexes down one in Wong's charts
It might also be a good idea to check over your indexes with the Excel spreadsheet that is on bjmath.com
Try to understand it is not the indexes you use, which can be a bit off with little or no impact, but which decisions you don't use. The i18 is not optimal in all conditions. Some index sets include indexes for decisions that a count is just plain not very good at. And some indexes are better canadates for rounding radically to match with others, while some, usually those with high corelations, should be used with as little rounding as possible.
I did mine by using Snyder's original algebraic approximation, and programed this into a range of computers, starting with a CPM box during a drafting course, then a Timex-Sinclair 1000, which was how the final Zen indexes for the 2nd printing of the first edition of Blackbelt in Blackjack originated (I originally sent Snyder a correction sheet and his indexes were algebraic done on a programable calculator). I then moved the program to an Atari 800XL, then an 386 PC, and then to a windows PC. I selected indexes by dropping out those with CCs less than 50% to avoid possible indexes that have the potential of actually costing money compared to basic strategy. This is unlikely, and only Norm Wattenberger of Qfit ever confirmed that such indexes exist, but it meant only about 7 indexes were dropped from the Zen charts I use, from those which had EORs available.
You can decide roughly, which indexes are more important, by taking Griffin's scores, in later editions of Theory of Blackjack, and multiplying them by the EOR CC, for your count. You can refine this by weighting them according to your actual spread.
But this all takes time and some computer knowledge. The better programs for this are being developed by T-hopper and chem nerd. They are the ones to ask for specific systems and conditions. The optimal i18 for spreading 1-12 in 6 decks, is different from the i18 for 8 decks with 1 cutoff, and is different still with Wonging 1-6, and with playing all 1-12.
My charts for the Uston +/- and the Zen, and Victor APC can be found by the link I have in the Books and Software page here.