Try this UkuleG, place the box on the table, put the neck down on the table touching the box in the position where you would join it. Measure the scale length from the nut onto the box. You will find that the bridge will be close to the end of the box. Now - move the neck away from the box to place it where it needs to be to put the bridge about 1 3/4" from the end of the box. The gap which you have made, between the joint face of the neck and the box, has to be filled, obviously. You can do this simply by inserting a block between the neck and the box. It won't look pretty, but the improvement to the sound will be well worth the extra effort - if the sound is important to you.
I hope I've explained that well.
Ukantor.
Edit - Yep, just as RonS says. Much better than my explanation! I've made about a dozen box ukes, but the shortest box I've used was 9 1/2", which is about ideal for a soprano joining at the twelfth fret.
