The two limits on focus items are:
You can't make a focus item that provides a total bonus higher than your lore
and
You can't stack bonuses (or the same type) from multiple focus items.
So, if I had Lore 4 I could make:
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+4 offensive fire control
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or
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+2 offensive fire control
+2 offensive fire power
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But not
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+3 offensive fire control
+2 offensive fire power
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Of course, I could simple make two different focus items, one that provided +4 offensive fire control, and another that provided +4 offensive fire power and have a +4 bonus to each.
What I couldn't do it make two +2 offensive fire control, + 2 offensive fire power foci and then stack them together.
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If you don't like how that works out, then you can just house rule that the maximum bonus you can get in any one category is equal to your lore, and allow stacking foci bonuses, or single mega-foci without too many problems.