Figuring Out Rigid Fork Length to Replace Suspension Fork

I'm considering replacing the suspension fork on my XC MTB with a rigid fork, as I'm starting to use it for more long-distance riding, and the conventional wisdom tells me that I'm losing energy due to the suspension fork.

The current fork is a 100mm travel coil suspension fork for 26" wheels (Example Picture: RockShox DART1). So, how do I determine the length of rigid fork to replace it with?