It depends on the Taper you want to design. Since you are checking abs(S12)^2, which is the refelction, it is normal that more number of modes means lower reflection. and it seems 60 modes might not be enough, except that the taper angle is large. Please refer to those links:
Convergence testing process for EME simulations
EME Convergence Testing - An Intuitive Approach
Understanding EME error diagnostics
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Please note that in some cases more number of modes may not necessarily be better.
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