The short version
Pick the requirement first; let the remaining XP solve the gaps around it.
XP is an allocation problem
The public system lets players invest XP into attributes. Different Race + Class combinations pay different costs for those investments.
Off-stat is a choice with a cost
You can build against a combination's natural direction, but reaching the same off-stat target may cost more and leave fewer total attributes at the same level.
Plan from requirements
Start with the equipment, spell or core skill that defines the build. Work backward to the attribute breakpoint it needs, then spend remaining XP where it improves the loop.
Do not average everything
A little of every attribute is easy to understand but often less efficient. Exact costs and breakpoints should be filled in after EA client testing.