Your theory assumes you are going to be taking a good amount of money right from the very beginning - enough to cover all your costs and then about another half again for yourself. This may be unrealistic in the start of your business, if indeed you are starting up your own business.
If you are already running your own business, a better way may be to work out all your costs per month, being generous about what expenditure you have and including any forgotten costs such as tax. You could then put that aside and any remainder is yours to do with what you would like.