Use your block heater. Under about 15 degrees, only a few diesels in perfect condition will start even with repeated extended glowing. Why tax your starter and engine?
We had 5 below this past weekend in Massachusetts, and I forgot to plug my wagon in. Ran the battery down trying to start it. Did the repeat glow, waited for the relay to click off, etc. Ended up pouring hot water on the block, which worked.
I concluded that the block heater is there for a reason. The next night, I plugged her in, and no problem, started right after the glow light went out. I plan to keep an extension cord with me for trips away from home in the winter.
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K