Doubts

Hello sir can u please clarify the following two doubts of mine

1) 5-10% of profit.b.t is acceptable range and is not material
But in some of the answers in bpp i havr noticed that when the mistatement is 7% of p.b.t they have considered it as material. I cannot understand why
Could you please tell me if there is any mistake in my understanding.

2) substantive procedure for completeness of purchases

For a sample of purchase orders trace it to GRN and purchase invoice and agree that the relative purchases are recognised in the purchase day book.. i have come up with this substantive procedure on my own.. could u please tell me if this is an appropriate procedure