That 20% may be based on the following tax court case Bindsell, TC Memo 1983-411. In another tax court case the property was deemed 100% personal use. Rental was $500 when fair rental was $600 based on a Reality firm's estimate. Jackson, TC Memo 1999-226. There are many other court cases with varying amounts of discounts allowed. However, that $2050 you cite probably can be reduced by the fair rental of that bedroom (probably using the area of the bedroom to the total area of the house).
This may fall into the rules that apply when you are renting out part of the home (assumes you'll not be using other parts of the house - akin to a motel, hotel or B&B. In this situation, expenses would have to be prorated between the area used by you (personal - non-deductible) except for the pro-rata portion of taxes and mortgage interest and the rental portion - fully deductible. the allocation is usually made based on square footage.
https://www.courtlistener.com/opinion/4555072/jackson-v-commissioner/
