Tektro r538 rear caliper... on the front?

I tend to favour 6 full turns of engagement - that threaded coupling is in tension and the only thing between you and smashing into the back of a lorry.

On the other hand, R538s are so cheap that it's hardly worth bothering

+1
Brake pivots are normally M6 x 1mm pitch which with a standard nut (5mm deep) means a minimum of 5 threads (or turns if you like) must be engaged. If the whole nut isn't engaged, the stress on the engaged threads is greatly increased, making stripping of the threads more likely. Bearing in mind the thread is in tension during braking, the whole set up could suddenly turn ugly when the caliper gets ripped out....