Re: Bulb Planting Depth
It's a guideline. Count the mulch, but in a fairly compact way. For instance cypress mulch can be applied a bit fluffy, but will reduce in volume/height before it starts to decompose. Don't count four inches in what will reduce to an inch.
Zone 6 will expect some freezes.
If you have bulbs for your area, that is purchased or bartered from people in your area and known to grow in your area, the bulbs will be hardier than what I can grow in Zone 9. You'll be planting your bulbs with the greatest mass of the bulb, the wider flare above the root nodes at about six inches below soil and mulch for a bulb that needs a six inch planting depth. This would probably be a substantial bulb, bigger than a golf ball, right?
The guideline in temperate zones (that's you, not me) will be based on the size of the bulb and on the species, but mostly the size of the bulb.
If you plant too shallowly, they may sprout prematurely during a colder season. A too shallow bulb may freeze, or burst, or be subjected to frost heave. If you manage to avoid those conditions, a too shallow bulb will probably have a much weaker stem and possibly damaged leaves with reduced flowering, and subsequent poor nutrition for the bulb because of inadequate photosynthesis in the leaves.
Planting depth will vary for your species. You aren't going to plant a crocus at six inches, but Narcissus spp. would be about right as I recall. But then I've been subtropical for a quarter century.