There's a lot of detail left out of your question, but I'd start with 110% and use Tinseth.
I suggest starting out with something that has a fairly wide range, like Pale Ale or Hefeweizen, to calibrate. Compare the taste to a known commercial sample. I use either Sierra Nevada Pale Ale (38 IBU, moderate cohumulone) or Widmier Hefeweizen (32 IBU low cohumulone). You should have it dialed in a batch or two.
Just keep in mind that the quality of your hops, yeast selection and ability to control oxidation downstream, will have a bigger effect on IBU perception than the calculation.