Reserves vary from building to building. Before deciding on an amount, I'd recommend you do a complete survey. How long will your boiler last? Your oil tank? How are your windows? Your fire escapes? Your facade? (Even if you're not covered by LL11, you need to make sure no liability issues are sneaking up on you in the form of loose bricks.) What infrastructure improvements (electrical, plumbing and waste disposal systems) are needed? How's your foundation? Your roof? Is your building dingy/dark/dirty? What's your mortgage? How are your arrears? Think of every question you will need to address in a financial manner.

Spend a little money upfront to get an engineer's assessment, then sketch out a timeline for capital improvements (boiler in 5 years, roof in 10, etc.) and other needed repairs. Try to build your reserves to cover these without an assessment.

If your immediate financial needs are high, you may need to boost your reserves quickly, through a maintenance increase, flip tax, or income-producing idea; you can also impose one-time operational assessments, such as a fuel-oil surcharge, if your PL permits.

Best of luck! R