I'm trying to calculate purchase cap rate of my older rental(s) in order to evaluate a new possible deal (8%).

All of my other rentals were purchased with cash, rehabbed and then cash-out refi.  For capital cost, should I use 25% of the appraisal (assume the refi loan was 75% loan-to-value)?  The reason I want to do this is because some of the previous deals I was able to refi more than my actual cost of purchase + rehab.  While it is nice to have infinite cap rate, not many deals like that are available.

I've hit the limit of properties that I can cash-out refi conventionally, fyi.

Yes, the WAY, weighted average yield, for the net yield, look to the weighted contribution of cash and leveraged funds to the total. Subsequent financing such as a second would be weighted to the total contribution and you'd consider it's interest rate. 1st at 7% is 75% of the total, 2nd is 9% at 10% of the total, 15% your cash at your assigned cost of capital of the total, compute to the weighted averages for the solution. If I followed what you're trying to do. You can also look to the WAM, weighted average maturities as loans are paid off or balloon and WAC, weighted average capital for use of cash. WAR, weighted average return. Probably goes further than you want to go. You're on the right track. :)