I don't think there is any exact science. During my last Starwood stay at Sheraton Pretoria, they authorised room rate plus ZAR 500 for my one night stay. The Sheraton Belgravia stay recently on a room rate of GBP 170 odd had a GBP 250 authorisation.
During my recent (infamous) Mauritius stay, Oberoi told me they will be seeking EUR 3000 on my card for a 4 night stay (roughly twice the room rate).
I had to pre-authorise charges for my mother-in-law's stay at Marriot Dead Sea last year as we were turning up two days after her due to flight cancellations. The nightly rate was USD JOD 450 plus taxes, they wanted to authorise JOD 2500 (explanation was in case she decided to use the spa or freak out on room service champagne) on the card I was using to guarantee her stay for two nights.