You're on the right track, 50% is a reasonable rule of thumb, however $/unit is more accurate. 

One year can not give an accurate idea of expenses.

But I am having difficulty learning how to do accurate comps.

I called my insurance company and got a quote for the lower coverage witch they went on and on about how accurate their numbers are for rebuilding, but when they finally provided a revised quote it was a few hundred dollars cheaper a year.Second, then I shopped around using my new replacement cost amount and found a company that was even a few hundred dollars cheaper than the revised quote from the company I used at the time. 

@Ashraf Martin it's a gamble on whether it's worth it or not.

In my area buying at auction is somewhat a gamble.

There are online tools you can use but I am not sure how accurate they would be given the number of units you have.  

You will need to run accurate comps to determine the ARV of the property.

One place is hard-money lending.Still, my concerns remain for hard money: try to get a couple deals under your belt so you can more accurately forecast costs and time frames. 

If they refuse, you will sue for everything owed and send them to collections so it sits on their record.It's a gamble but it works most of the time.