our parts are all soft silicon with no real flat surfaces, datum B is a tangent edge that has a flat horizontal area about an inch long. I agree, not ideal but it gives the measurement group a place to orient
This is so basic I feel shameful but there is a big debate at work right now.
Incase the attachment doesnt work, the picture shows a blob with a Profile tolerance of .5 all around it constrained to a parrallel datum A only. one of the surfaces of the blob is Datum B with a 2.5" basic...