Symmetry Plane / Base Align

[[JM...]]

Alignment question. B1 & B2 are not coplaner surfaces. They are +.050 and -.050 on each side of zero according to the model. Would you measure both planes and then create (2) Theo planes with recall points and offset the actuals -.050 and +.050 and use symmetry plane for secondary alignment B? Wasnt sure if you could use symmetry plane with the actual measured planes and if that might skew things. I know there is the feature OFFSET Plane but I've only use that if I have 3 stepped planes for my primary datum.

Thanks in advance.