I agree with Pirl... we just cut ours back to two feet off the ground a week ago. it runs up a trellis/fence (pic below), and rises above a few large Hydrangeas, which is why we don't cut them lower to the ground. new buds were beginning to swell so we had a very good visual as to where to cut. go out and take a look at the stems about a foot off the ground. or maybe wait a few weeks to do so. look for a good concentration of swollen buds on the lower stems. the more the better. cut just above them.
not sure what you mean by, "this will leave no flowers this year". it shouldn't be flowering right now anyway and Jackmanni clematis only bloom on this coming years new growth. if you were to cut it back now, or in a few weeks, you can expect it to be in full flower by late June or early July.