I am familiar with Calvert's short term financing.
They way they analyze deals can make them seem tough; If margins are tight, they will not fund the deals. If they approve the financing, there's a good chance that there is money to be made on the deal.
Expect 10-12% interest plus 2% setup fees up front. Only real advantage of Calvert over other private lenders is that they have a flip product that only needs $10k down (theoretically leaving more cash in hand to fund your renovations).
There are better lenders/better terms/better rates around, but they typically want 20% down (instead of the $10k.)
PM me if you'd like to chat, or a referral to a mortgage broker who uses comparable lenders