Different cells can react quite differently to various selection antibiotics. However, determining the right concentration is pretty easy and straight forward. Just plate you cells into a 96 well plate and test your different concentrations in replicates. Since I recall MDCK to be pretty robust I would check concentrations between 100 and 1000 µg/ml. Be sure to include a control without Zeocin. Since I always suffered from edge effects, I would not pate any cells but only medium into the outer wells.

You can also try your transduction in parallel. Ideally, you have a reportervirus at hand, for example expressing EGFP. In this case, I would use 30 wells with untransduced and 30 wells with transduced cells (only 60 wells remain if you avoid the outer wells of a 96 well plate). If you plate three rows each, you can check the different Zeocin concentrations in the different columns. Thereby, you can check each condition in triplicates in one plate. Start the selection one day after transduction. Make sure to renew medium and selection antibiotic every 2 - 3 days. Check for a concentration where your untransduced cells die and see if you find surviving cells in the corresponding wells with transduced cells after about 7 to 10 days after selection has started. Then check expression of EGFP under the fluorescence microscope. Keep in mind that expression levels are lower after transduction as compared to transfection, so your cells might be a bit dim.