make sure you have proper antifreeze/water ratio, make sure block heater is installed correctly. make sure you run properly sized extension cord.
for myself a "battery blanket / warmer" is on todo list as well. lower the temp, the lower max charge a battery can hold, or i should say can produce/hold. (someone else can explain better)
i do have a glow plug as well.
with temps hitting -10 F a couple days of the year, and normally a ugly snow storm / ice storm accompanies those low temps. tossing a "cardboard" over front of radiator, or tarp or something, to help reduce wind blowing through radiator, as block core heater warms things up helps. (Not able to place machine in shed)
personally prefer to turn heater on night before, and just leave it on, it might be a waste, in electricity bill, that little bit of heat, might help keep diesel from gelling up, if fuel tank is in a good spot, more likely to warm all the metal up all the way to the center of the engine. and to the opposite side that the block heater is installed on. it might be very limited, but tiny bit of heat to the oil down in the oil pan.