Working backwards...
5-6 slots is a max, not an availible range.
You're probably reading it wrong. You can have power and control on the same item. The numbers on power compared to the numbers on control don't have to match. The numbers on power have to match and the numbers on control have to match. Examples:
+4 power to air costs 4 slots and is fine.
+2 power to water/air costs 4 slots and is fine.
+2 power to air, +2 control to air costs 4 slots and is fine.
+2 power to air, +1 control to water/air costs 4 slots and is fine.
+1 power to water/air, +1 control to earth/fire costs 4 slots and is fine.
+1 power to air, +2 control to air, +1 control to water costs 4 slots but is an illegal item.
Sorry, but your examples are partly wrong.
You
can have both Control and Power one one item. Power and control do
not have to be the same value.
So far your are correct.
However, the bonuses have to apply to
all elements on the item equally.
If one really needs a formula, or a step by step way to construct a focus item, then the easiest way is to
1. decide which bonuses you want.
2. chose the elements you want
all those bonuses to apply to
Cost is calculated by adding up the different bonus values, and multiplying by the number of elements.
This final cost can not exceed your lore for any single item.
For Evocation Foci its:
(offensive power + defensive power + offensive control + defensive control) * (number of elements covered) = Cost
For Thaumaturgy foci its
(Power + Control) * (number of areas covered) = Cost