I'm assuming the mode is set to s on the bottom right corner and the portamento set to 5 on the dial for your sylenth. If you want a single continuous legato transition with absolutely no break in between notes, then it's best to have all the notes overlap each other and have mono legato activated. By doing so, it will be like one long note being played with smooth sliding between pitches. The portamento knob adjusts how quickly it takes to glide between notes so lower number would mean faster glide and higher number would result in a very slow glide between pitches. However, you can still achieve a glide without notes overlapping. As long as the mode is set to s (slide) and portamento dial is above 1 you can achieve a glide sound effect. The only difference with no overlapping midi notes is you will also hear the attack/transient portion of every note played when the glide takes place. It will be like hearing separate notes being played but a glide transition will occur between them. Experiment between the 2 and see which works better in context with your melody. You can always mix and match to achieve unique slide variations. Hope this helps :D!