Unlikely, since it is working as intended. I'm going to kick this over to the Modding forum where folks might be able to help you implement what you are after.
Afaik, opcode 60 (Miscast Magic) uses a plain flat value modifier, i.e. it sets the percentage of miscasting spells and isn't cumulative. Specifying a negative value doesn't make sense as negative values always behave the same as 0%. It might be possible to override a previously set percentage with a new one, but I haven't tested whether this actually works.
I have tested this with making a token. Made my leather armor 5% miscast, added a dagger that was 1% miscast and it went to 6%. Added 257 and it went to 7.
It most certainly increases, but does not decrease.
Looks like you are right. The opcode descriptions I've found were rather vague in that regard. This might be a good candidate for a feature request though.
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It might be possible to override a previously set percentage with a new one, but I haven't tested whether this actually works.
I have tested this with making a token. Made my leather armor 5% miscast, added a dagger that was 1% miscast and it went to 6%. Added 257 and it went to 7.
It most certainly increases, but does not decrease.