I own multi-family in Brooklyn and some single family in the Midwest and I can tell you the 50% rule does not apply to Brooklyn. If I had to put a number on expenses prior to P&I expenses it’s probably around 20-25% of gross rents for my three family properties, but keep in mind that whether you have you or your tenants pay utilities, whether you have water efficient fixtures, you self-manage, etc all play a part in this. For your first deal, I HIGHLY recommend you put numbers into a spreadsheet and show it to someone before pulling the trigger to ensure you don’t miss anything (if you’d like I’d be happy to take a look at them just PM me).
Regarding repairs, it’s hard to predict, but I’ve found that using a fixed number (based on the condition of my property) has been more accurate for me. The way I see it, it’s going to cost me the same amount to send a plumber out whether the monthly rent is $3,000 or $800. As such, I’ve found the 50% rule is more applicable in more affordable cities than New York.