What is the best temperature for bending mahogony on a bending iron?

No ideal - each piece of wood is different.

What I do is place the wood on the hot pipe, apply *gentle* but firm pressure about 3 ins either side, and rock the wood (to prevent heating just one spot, reduces the chance of scorching). When it's hot enough you will feel it start to move. Then persuade, rather than press, it into the shape you want.

If you have an offcut, practice on that first to get the feel.
 
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