As an organic synthesis book unfurls, it just show basic synthetic methods without operating temperature, the supposed maximum concentration of 1,2-dibromoethane and other details. So I hope an anwser.

By the way, the translation of text in picture is: Add ether disolving 1,2-dibromoethane dropwise to magnesium scrap with fast reaction and heat generation. 1,2-dibromoethane transfers ethene and leaves, and magnesium dibromide generated in such way dissolves well in ether. And attention, if the concentration of MgBr2-Et2O complex goes too high, it will layer with ether