@Michael A.
i see a lot of posts on BP where people get confused because they try to calculate using conventional financing like you did. if you use financing, you pay off the loan you took out.
so in an example where you buy for 100K using hard money with, say, 20% down and 40K rehab in your own cash, appraises at 200K:
-75% of 200K is 150.
-use the 150k to: pay back 80k loan, 40K rehab, and 20K down payment to yourself... but there are also holding costs, like insurance and utilities; financing charges on the refi; and interest on the HM. these costs are substantial and always overlooked.
-HM is really expensive. you'll pay points up front to get the loan and then monthly interest payments.
and... if you buy using conventional financing... it probably doesn't work as a BRRRR. you can't go on the MLS, find a random property, buy with conventional financing, and BRRRR. it's not going to work.
and there aren't too many rules on your questions. BRRRR works on any property of any type, SFH, small multi, big multi. the difference between SFH and big multi is that the ARV of the SFH is going to be based on comps, and on big multi it will be based on cap rate. so on a SFH the thing that matters is boosting the ARV via the rehab, which you hope will increase the rent to cover the debt service when you refi. if you can't boost the ARV it's not going to work.
for awesome deep dives from a pro, watch Tarl Yarber's YouTube videos. he gets way into the weeds on the numbers and doesn't omit anything.
hope this helps