Conversion from a crude ¹H NMR with no internal standard, using cleanly resolved, non-overlapping, non-exchangeable peaks.
Material lost to decomposition never appears here, so conversion can read high when the actual yield is low. For a real yield, use qNMR with an internal standard.
SM vs product — when you have one clean, resolved product peak with a known proton count. This approach assumes starting material goes only to that product, so an unassigned side-product will make it under-read.
SM vs reference peak — when a signal cannot be directly assigned to product. Reference against a proton environment (e.g. a remote tert-butyl moiety) that the reaction leaves completely untouched.