I need to speed up the video at exactly 1.5x speed. what value should I put in time strech window?

You have three ways to do this. My preferred way is to select the footage in the Project Panel, Interpret Footage (Alt/Option + Ctrl/Cmnd + g), and then multiply the frame rate by the speed increase you want. If you have a 6 second clip and the frame rate is 29.97 and you click on the 29.97, press the right arrow, and type * 1.5 the clip will now be four seconds which is one and a half times faster. If you Divide by the speed increase the clip will now be 9-seconds, which is one and a half times slower. This changes the playback speed of the entire shot.

If you want to use Time Remapping and you have trimmed the in and out points of the shot in your comp you need to Pre-compose, move all attributes to the new comp, and trim the Pre-comp to the layer length. Then you adjust the timing of the last keyframe by moving it up or down the timeline. The same math holds, but it is easier if you change the time display to frames by Ctrl/Cmnd clicking on the timecode display. If the original time remap keyframe is at frame 900 then 900 * 1.5 is 1350 or 50% longer and 900 / 1.5 is 600 or 50% shorter. 

You can also eyeball things with time remapping if you open the graph editor, select edit speed graph, then drag the last keyframe to the right to slow down the clip and to the left to speed it up. You can see the seconds per second scale and drag the keyframe left or right until the speed is 66* or 150*

My usual way to handle time changes is to set the outpoint where I want it to be, then use time remapping to make the clip fit the hole in the timeline. It tends to be more accurate for most of my projects.

You can let After Effects do the math in the Stretch column in the timeline. 100 / 1.5 is 66.66666 which is 1.5 times faster. 100 * 1.5 is 1.5 times slower.