What percentage of those forced to relocate actually died as a result of the relocation?
In "Armenia: the case for a forgotten genocide", 1972, Dickran H. Boyajian quotes the deputy director of the settlement of refugees, who in 1916 said that 10% of refugees arrived at their destination.
US Counsel Jesse B. Jackson reported that 85% of the deportees died in one of his official reports.
Johannes Lepsius wrote two reports where he stated that 90% of the deportees died.
Talaat Pasha's official report on the genocide doesn't include numbers about the relocation, just a census over Armenians, and says that there was 1,256,403 Armenians in 1914, and 284,157 in 1917. That gives a total death rate of 75%. Not all Armenians were forced on death marches, and many Armenians were killed in other ways, so this doesn't say much about the death rate for the relocations, but it's clear that it's quite high.
Mark quotes Arnold Toynbee as saying that it is at least 50%.
So estimates on the percentage of dead ranges between the very conservative 50% and up to 90%.
According to Arnold Toynbee at least 50 percent [500,000 - 700,000] would be casualty of the deportations.
Wikipedia, which references Arnold Toynbee, "A Summary of Armenian History up to and Including the Year 1915," in Viscount Bryce, preface, The Treatment of Armenians in the Ottoman Empire 1915-16: Documents presented to Viscount Grey of Fallodon, Secretary of State for Foreign Affairs By Viscount Bryce (New York and London: G.P.Putnam's Sons, for His Majesty's Stationary Office, London, 1916), pp. 637-653.