Choose joist direction
The deck dimension across the joists controls how many joist lines are required.
The joist run controls member length and possible stock-length splices.
Count spaces before members
Divide the supported width by the maximum layout spacing and round spaces up, then derive the required joist lines.
Keep the assumption visible so it can be replaced when project or product data is available.
Add special framing separately
Stairs, guards, picture-frame decking, beams, openings, and concentrated loads can require doubled or additional members.
Do not hide these pieces inside a generic waste factor.
Check stock lengths
A linear-foot total does not guarantee a workable cut list.
Match required runs to available lumber lengths and permitted splice conditions.
Do not use takeoff spacing as design
Allowable joist span depends on species, grade, size, loading, treatment, and code conditions.
Use an applicable structural design reference before fixing the spacing or span.