aftermarket speedometer installation

I have bought a speedo from China (very cheap), since my used desperado came with none. I was able to wire everything but I can't decide from where to get rpm signal. 
Everyone says that I should get it from the coil. the problem is that my service manual says that peak voltage at coil is more than 200 volts and the speedo says it can withstand 100 volts.
I guess I should spend some more money to get a better speedo. Did anyone installed aftermarket speedo to a Suzuki vz400 or vz800 ?