It's not rocket science but many people do seek out convoluted alternate methods for no apparent reason.
The idea of using flight time + x minutes is approved for when UK military pilots are translating their military logbooks into civil hours. Under military regs only flight time (t/o to ldg) is logged. The CAA allow specified additions to allow for taxy time.
For civvy pilots just log off blocks to on blocks. Simples. Any other method that the flying club uses for billing is irrelevant.