This is not a rule of thumb, but at a point in the past i bought a new primary house - while renting out the "old" house, which had a conventional [owner occupied] mortgage on it. The mortgage broker for the new house wanted to see 2 years of taxes, showing that i claimed the income, in order to use 100% of the rental income on the old house, as part of my total income.
As i did not have 2 years tax / rental income / depreciation reciepts, they would not allow for the full amount to be claimed as income -- However, they verified what i was charging per the square footage was reasonable for the town, and agreed to use a figure of 75% of the rent, as income.
Maybe your mortgage company / broker would allow you a similar percentage [ doesn't sound like you have been renting for 2 years yet].