Living Arrangement - Rental Agreement

Hello everyone. I hope this is not a duplicate. I just typed this post...and lost it when I was almost done. Okay, my 18-year-old daughter was just approved for SSI. She has an autism diagnosis and lives at home. She will not be paying her fair share of the household expenses because our mortgage is high and she would lose 1/3 of her potential SSI amount. Instead, I am preparing a rental agreement. I have researched the FMV for rentals in my area and also researched my county on the HUD website to see what they consider FMV. The amount is between $350-$450. She will be paying for her own food. I would like to separate out the utilities from the amount of rent for the room. I am not quite sure how to do this? Do I need to determine the FMV of the utilities for that room? That would be difficult. Can I state on the rental agreement that the rent amount is $350 and utilities will be an additional $50 per month for a total of $400. I am concerned that they are going to ask me how I arrived at the amount for the utilities. If anyone has any thoughts on how to do this and how much I should charge for utilities, I would be very grateful if you would post your thoughts. Thank you.

Living Arrangement - Rental Agreement