Connect the tiller to the lower lift arms. Find a dip or dig a small trench that approximates normal tilling depth. Lower the tiller into the dip/trench to simulate the tiller being at normal operating depth.
Depending upon your draft control capability, tie a string to one of the toplink bracket holes on the tractor. If you don't have draft control, use the pin hole of your choice. If you DO have draft control, select the hole that activates the draft control linkage.
Then run the string back to the general area where you plan the new bracket. Raise/lower the string until it's parallel with the lower lift arms. When the string is parallel, that's where you'll want the implement end pin holes to be. Plan your new bracket around that location.
If your tractor doesn't have draft control, this might be the right time to consider a floating toplink bracket while you're at it.
//greg//