How much waste should I allow when ordering stock?

Don't add a flat percentage. Work out the cutting plan first, order the bars it uses, and add a spare only as cover for a miscut. A 10% or 15% allowance is a guess about how well the parts will pack, and the real waste depends entirely on the cut list: 2.9% on one job below, 25% on the other. A flat allowance buys steel you don't need on the first and leaves you a bar short on the second.

Job 1: the allowance over-orders

140 parts from 6000 mm bars with a 3 mm blade: 40 at 2600 mm, 40 at 1650 mm and 60 at 1050 mm. That is 233000 mm of parts.

The plan cuts every part from 40 bars and is proved optimal: 20 bars cut 2600 + 1650 + 1650, and 20 cut 2600 + 1050 + 1050 + 1050. 2.9% of the steel bought is left over. The 10% rule buys 3 bars, 18 metres, that go straight onto the rack. The 15% rule buys 5.

Job 2: the allowance under-orders

Twelve parts from the same 6000 mm bars: six at 3100 mm and six at 1400 mm. That is 27000 mm of parts; add 10% and you get 29700 mm, which is 5 bars.

It takes 6. Two 3100s plus the cut come to 6203 mm, so no bar holds two of them, and every 3100 needs a bar of its own. The optimiser's plan is six bars of 3100 + 1400, leaving 1494 mm on each: 25% of the steel bought. Order on the 10% rule and you are a bar short with the saw running.

Where an allowance still belongs

Allowances are for what the plan cannot see: a miscut, a bar that arrives bent or short, rough ends that need squaring. Those are per-bar risks, so cover them per bar: a spare bar for the miscut, and the trim setting, which takes a blade width off both ends of every bar, for the ends. Put them on top of a real plan, not in place of one.

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