One way to go about it is to find a comfortable position to sit or stand and measure your eyelevel from the ground. Subtract from this the height of the mount from where it would join the pier to the position of the eyepiece when your telescope is at the lowest practical observing position and also when the telescope is at it’s highest practical observing position. That should give you an idea of the pier height. depending on your interests and location you might find there’s a certain ‘band’ of altitude your favorite objects reside in I’m thinking of planets or the Moon which will make choosing the pier height a bit easier.
Another method may seem a bit odd. But if you get a pole higher than the top of the sides of your observatory you wedge it in the vertical position. This simulates the pier. Next take another pole which will simulate the telescope and tie it with string to what you think is a suitable height to the vertical pole. You then can pivot the pole simulating the telescope up and down to find out if the observing height is practical. If not just untie the ‘telescope pole’ and move it up or down the vertical pole until you are happy with the range of heights from the viewing end where an eyepiece would be. Measure on the vertical pole the point where the ‘telescope pole’ is to the ground and subtract from this the height of the mount and you will arrive at your desired pier height.
If you change your telescope you might have to put an extension piece on top of the pier or use steps.
I hope this helps