We've welded PODS on about a 1/2 dozen boats over the last (2) two years.
Depending on the motor being run, a good gauge is the same as installing TRIM TABS on a boat.
Use anything flat/straight (yard stick/2x2/etc) & place it on the bottom of the boat so it sticks past the transom & PAST where you want the pods to end.
MOST IMPORTANT, from their the pods should begin @ the edge of the transom/bottom & angle UP 1/2 - 3/4 of an inch.
This way when in REVERSE the boat will semi plane & weeds/mud/muck will run underneath the PODS/BOTTOM & not get hung up around the transom.
Also, it would not have you PLANING so flat when up & running but will still afford you the stability when running through the shiat.
We run alot of hydrilla, hyacinths & of course mud & the BACK ANGLE is the entire trick to installing pods.
BTW, the flat bottom/no chine hulls like the GD, PD & GT are the toughest to make fit due to the curve of the transon/gunnel.
We use a torch with a sooty flame to form the curves as we tack it to the hull.
Good Luck!!!