To make concrete forms for a shed foundation end at an even height.
Place the forms on the ground where the supports will be. Draw a level line on the form of the support that will be lowest and cut all of the forms down to the height of that line.
Place the forms on the ground where the supports will be. Draw a line on one of the forms and cut all of the forms down to the height of that line. Use sandpaper even out the height of the forms.