The constant selection only affects the selection and has no effect on the order in which the aggregation and formula calculation are processed.
The system does not support expressing percentages of aggregration reference characteristics with a formula that defines the reference point with constant selection.
How many products have revenue of > 1% of the total revenue?
This question could be expressed as follows:
Total revenue: Revenue with constant selection on characteristic Product
Relative revenue = Revenue / total revenue
Number of relevant products = relevant revenue > 0.01 with aggregation TOTAL over characteristic Product
This example would work if the aggregated total revenue went into the relative revenue. As the constant selection only affects the filter however, the relative revenue in the example above will always be 1 if revenue is > 0. This means that the number of relevant products is identical to the number of all products that data has been posted for.