The Carbon to Nitrogen ratio of horse manure is ~30:1 which is ideal for composting (C:N ratio will be higher for a mixture of manure and litter). The C:N ratio of sawdust ranges from 200 to 500:1. So for composting horse manure and sawdust you mainly need the N source to balance out the high C:N ratio of sawdust.  

To do the calculation for you, I used assumptions from WSU’s Compost Mix calculator https://puyallup.wsu.edu/soils/compost-mix-calculator/

Assuming the C:N ratio of sawdust is 400:1, carbon content 60%  and bulk density of ~400 pounds/cubic yard, one cubic yard sawdust will contain 240 pounds of Carbon and only 0.6 pounds of Nitrogen

For ideal composting conditions, for every 30 parts of Carbon you need 1 part of Nitrogen i.e. one cubic yard of sawdust will need 8 pounds of N 

Your 1 cubic yard sawdust already contains 0.6 pounds of N so you need to add 7.4 pounds of N or 16 pounds of urea (Urea contains 46% N)

Based on above calculations with the bulk density and carbon content assumptions I used, every cubic yard of sawdust (C:N ratio of 400:1) will need 16 pounds of urea. If you are using a mixture of 20% horse manure and 80% of this sawdust in one cubic yard (3ftx3ftx3ft) compost then you will need 12.8 pounds of urea/cubic yard.

I hope this calculation gives you a good starting point to make adjustments in your manure to sawdust ratio to keep your composting cost effective.  Please see below the links to further information on adjusting C:N ratio for horse manure composting, layering and care of your compost pile and OSU Small farms program's composting resources webpage link

http://extension.missouri.edu/p/g6956

http://www.acrcd.org/Portals/0/Equine%20Fact%20Sheets/Equine_C-N_Ratio.pdf

http://smallfarms.oregonstate.edu/agricultural-composting-and-water-quality-resources

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