Run these checks
The target costs more than your current Cash
That is the normal use case. Enter the full target cost; the planner subtracts your current balance before estimating time.
Your Cash rate keeps moving
Wait for one stable state, avoid changing decorations mid-sample, and record the number tied to that setup. Recalculate whenever the displayed rate changes.
Two upgrades look equally useful
Compare them one at a time using visible cost and the observed result after purchase. The public data does not support a universal optimal build order.