The angle problem
An anchor is designed to dig into the bottom when it's pulled at a shallow angle, close to horizontal. The shorter your rode relative to the water depth, the steeper that pull angle becomes, and a steep pull tends to break the anchor out of the bottom rather than set it deeper. Scope ratio is just a way of guaranteeing that angle stays shallow enough for the anchor to actually hold.
Why bow height counts
It's easy to forget that the rode doesn't start at the water's surface — it starts wherever it leaves the boat, which on most fishing boats and pontoons is a few feet above the waterline. That extra height adds to the effective depth the rode has to span, which is why the calculation uses total height (water depth plus bow height) rather than depth alone. On a small jon boat this difference is minor; on a pontoon with a raised bow rail it's enough to matter.
Picking a ratio for your situation
5:1 is a reasonable minimum for a short, calm-water stop where you're staying near the helm and can watch your position. 7:1 is the standard recommendation for a normal overnight anchor or extended fishing stop in typical conditions. 10:1 or more is worth using when wind, current, or boat traffic wake is going to be putting real load on the line — river anchoring for walleye in current is a good example where the extra scope earns its keep.
Chain versus rope
All-chain rode lies flatter along the bottom under its own weight, which effectively reduces the pull angle at the anchor even before you account for scope ratio. That's why boats running all-chain can often get away with slightly less scope than the same boat running a rope-and-chain combination. It's a small effect compared to scope ratio itself, but it's part of why two boats anchored in the same spot with the same length of line out can behave differently.
Two-anchor setups
Anchoring bow and stern — common for staying parked over a specific piece of structure while river fishing — changes the math because each anchor is sharing the holding load and isn't fighting boat swing the way a single bow anchor does. Many anglers running this setup use slightly less scope on each line than they would for a single anchor, since the two lines are working together to keep the boat in place rather than each one needing to independently resist the full force of wind and current.
When scope alone isn't the fix
If you're running proper scope for your conditions and still dragging, the next things to check are anchor type for your specific bottom (sand, mud, rock, and weed all favor different anchor designs) and anchor weight or size relative to your boat. Scope ratio assumes the anchor itself is appropriately sized to begin with — it can't compensate for an anchor that's simply too light for the boat and conditions.