Determining maintenance budget for HUD property.

I’m developing a property with 1 main structure (6bd) and 2 ADUs (3bd). They’re permanent HUD structures (affixed like stick-built). The structure cost is only 300k, but the total finished cost is ~802k due to land and prep in an expensive area. Income will be ~$119,400/yr through Section 8 (the payment tier is above market here). Debt service is ~$67,000/yr after taxes and insurance.

I’m losing my mind trying to guesstimate operating expenses. Many recommend 15% of income for maintenance/vacancy, which is ~$18k/yr. I'm having a hard time imagining I'd need that much for structures whose net value is only 300k.

Here's my math: The 1% rule on 300k is 3k. Even if I double that to 6k to account for site maintenance, plus ~$2,400 to cover income loss for vacancy (actuarily fair amount using the average 0.02% vacancy of section 8) my total is only $8,400/yr.

I know I probably need much more than $8,400, but I can figure out *how* much more. What would you guys recommend budgeting for this scenario? Any advice is appreciated, even if it’s just calling me an idiot because I didn’t xyz.

Thanks.