When designing with roller chain it helps to design for the specific center distance to eliminate the need for chain tensioners.  How do you calculate this?  Thankfully there are chain calculators on the internet that you can use.  One such calculator is located here:
Roller Chain Calculator

#35 roller chain has a chain pitch of .375 inches.  Put in the sprocket sizes you need to achieve the reduction ratio you want and that are also available.  Then put in the distance that you have available (most likely you will get this from your CAD model).  This will give you the count of links needed.  From there you can use the top calculator to round down (or possibly up) to the nearest even number of links.  That is your center distance.  Use this calculated distance in your CAD model to correctly space your shafts so you don’t need to use chain tensioners.

Sprockets can be found at:

• AndyMark Sprockets
• VexPro Sprockets
• McMaster Carr Sprockets