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ultra sound sensor placement on robot SRF 04 SRF04

Two SRF 04

Hello again. I ordered those two without any additional servo motor, so I have to put them to dedect left and right.

 How can I calculate what is best angle to go with? Or design? Also i have two DC motors with gears and stuff, they work well. 

  I was thinking one front and one back, but if I had atleast one servo. Or two in front, everyone in their angle.