I'm going to stop you for a second and say the worry should be focused around the return OF capital and then the return ON capital. You need to have certain safeguards in place and price the loan according to the risk involved. For example, a borrower that only needs 50% of the ARV on a cosmetic rehab in my market is far less of a risk in the loan that someone who wants 75% and is then bringing in more of their own capital for the rehab, so maybe make $5k off the deal, if everything goes smoothly. So I would price the first loan lower (given all the other parameters of the loan are same) than I would the second one.
Things to do to mitigate your risk on LOSS of capital (hence return OF capital ) would be:
1) All money flows through the closing agent/attorney. The borrower signs docs there, they record them, you get some originals back after recording and some after closing, and the funds provided get wired directly to the closing agent, NOT THE BORROWER.
2) Make sure there is adequate insurance on the property. You want replacement cost value (RCV) policies as opposed to ACV policies. RCV will give today's dollars in replacement cost, versus the other option which takes into account depreciation of the assets being replaced (so hence lower payout if something happens). You are lending on the structure AND the land, but you can only get insurance for the structure. Make sure that structure coverage is enough to at least cover your loan, preferably more. Get added as a mortgagee/loss payee to the insurance so if it lapses or there is a claim, you are notified. In the case of it lapsing you can then force place insurance to protect the asset.
3) Get lenders title insurance (the borrower pays for this as well). Read the policy, make sure you understand are comfortable with the exclusions. IF you don't understand it, ask your title rep or attorney familiar with lending.
4) Get loan docs professionally drawn up by someone familiar with lending in your market. DO NOT use templates you found off the internet. Most states have bare minimum information and language that has to be included in lien documents in order to be enforceable, so you don't want to lose all your capital just because you didn't feel like paying an attorney $1000 to draw up documents protecting 100k+ plus in capital. PS. This can also be added on to the borrower to pay.
5) Know the USURY limits in your market. There are many states that have upper limits on the amount of interest and fees. You again don't want to be over that and have your loan declared illegal. You could lose your capital or have to pay back all the interest the borrower paid you over the course of the loan. Do not mess with the usury laws!
6) Get scope of work for the project, have them supply live video walk throughs if you are not local to the property. You can google walk the neighborhood as well to get a feel for the area if you aren't local. Make sure that scope of work makes sense for the property and the area. Over improvement is a real thing! THe last thing you want is a borrower that spends too much on a property, and now they have no reserves to do a refinance into conventional financing or the market softens and they have to sell at a loss (and hopefully not your loss because you aren't lending all the way up to 100% LTV).
7) NOW you can think about interest rates. Again, this is going to be widely different from market to market, and potentially loan to loan because no two loans are the same. First lien mortgages are usually 8% to 14% from most private lenders I know personally. For those that do 2nd lien loans, those are costlier (because of the higher risk) and they are generally starting at around 14% and just go up from there. Again, private lending if very flexible so don't take these ranges as gospel. Protect yourself first, and then worry about the return.