It's a bit of a gamble on how long an accessory change can take too, normally easy and quick until you find some where the original has been really badly done, short cables in sockets then L&N the opposite way round on the new ones,metal switches that need a new earth link from the backbox, plate screws cross threaded then rammed in, fun when they shear off taking them out etc etc. I'd look at the entire job in terms of days then allow a bit on top for snags, having said that though the OPs target hourly rate seems high to me.