I would probably specify in-situ testing in a variety of panels with a few different conditions (e.g. some centered, some closer to edge, etc.). That's our usual course when we're outside published tables, which is not an infrequent occurrence. If you don't have that opportunity at the design stage, then increase number of anchors by some margin and write the testing into the contract. Be sure to let the owner / client know that the spacing is contingent on satisfactory field performance and that if the field test does not pass muster, you may not be able to do what they envision. Also, to avoid cancelled construction contract fees and all that, write it such that a mock-up (with field testing) occurs prior to everything else and that the contractor shall include in their price the possibility of termination due to lack of feasibility or similar.