Breaking down the formula
Why two stages? Recommended Qty always shows the true target position, regardless of what's already inbound - so a buyer can see the real need at a glance. Purchase Qty then nets that against any open PO, so it's the actual number to put on a new order.
Safety Stock in the target: Recommended Qty always includes Safety Stock as a floor. This guarantees every SKU flagged Out of Stock, Below Safety Stock, or Reorder Now gets a nonzero recommendation - closing a gap where urgent SKUs used to show no purchase suggestion at all.
Review Cycle: How often you look at each class. A-class SKUs (high value) are reviewed every 7 days. You are ordering to cover Lead Time + Review Cycle days of demand.
Movement Profile: How fast a SKU sells. Fast = over 1 unit/day. Medium = 0.1-1/day. Slow = under 0.1/day. Slower movers get a larger buffer because their demand is harder to predict.
Buffer: The multiplier above 1.0 adds a safety margin. Medium = 1.20 means you order 20% more than the bare coverage window to absorb demand variance.
Netting open POs: Already-ordered stock is subtracted in Stage 2 only, so it never hides the true target computed in Stage 1.
Real example - Carbon Needles .20x25 (CB5.20x25):
Inputs
Class: B | Velocity: 4.3/day | Movement: Fast
Lead Time: 15 days | Safety Stock: 267 units
Available: 158 units | Open PO: 0 units | Pack size: 100
Stage 1 - Recommended Qty
Review Cycle (B) = 14 days | Buffer (Fast) = 1.10
Target = CEIL(4.3 x (15 + 14) x 1.10) = CEIL(137.2) = 138
Recommended Qty = 267 + 138 - 158 = 247 units
Stage 2 - Purchase Qty
Raw = MAX(0, 247 - 0) = 247
Rounded up to nearest 100 = 300 units
Recommended Qty shows 247 (the true target). Purchase Qty shows 300 - the number to actually place on the next PO.