Borrow, rent, or buy the proper torque wrench. The wrench should also make the torque value somewhere in the middle of it's range if possible. As you get toward the minimum or maximum of the range, the torque will be less accurate.
Bolt clamping force is based on the bolt stretch. You can calculate the required torque, because you know how much the bolt will stretch in say, 180 deg. It will rotate 1/2 thread, and depending on thread pitch, it will stretch the bolt a certain amount, and provide the necessary clamping force.
But, you still need consider the material of the bolt and what it's threaded into.
Just borrow a torque wrench.
PS: I would be leery about how well taken care of the auto parts store tool rentals were. Find a friend, or take it to a shop for final torque