You need a new property manager.The "50% rule" is a rule of thumb that say of the gross scheduled rents, expenses, vacancy and capital expenditures will eat up about 50%. 

I would have all your major capital expenditures evaluated to find out how much life expectancy is left on those items.

How do you pay the other expenses; repairs, maintenance, property management, capital expenditures?  

Make a spreadsheet and take your total rent and subtract vacancy, mortgage, capital expenditures/Maintenance, Tax, insurance, and property management (You always want to include this just incase down the road you end up using property management).

that doesn't account for vacancies, or capital expenditures.

However, once you move out, your cash flow should be around $350 - capital expenditure/month. 

Once you have enough cash to buy a home cash + rehab + capital expenditures, you can BRRRR/flip over and over. 

Please offer your advice on the best place to purchase these large capital expenditures.

I am allocating roughly $100/month per unit to go towards repairs and capital expenditures. 

Debt is allocated by tracing disbursements of the debt proceeds to specific expenditures.