40% of the depreciable basis needs to be placed into service in the last quarter for the M-Q convention to apply
where did you see 15%?
The mid-quarter convention. Use this convention if the total depreciable bases of MACRS property you placed in service during the last 3 months of the tax year (excluding nonresidential real property, residential rental property, any rail-road grading or tunnel bore, property placed in service and disposed of in the same year, and property that is being depreciated under a method other than MACRS) are more than 40% of the total depreciable bases of all MACRS property you placed in service during the entire year.
