How to par cook potatoes for use as home fries
I wouldn't deep fry potatoes to par cook them for use with anything other than future frying. That cooking method gives the potatoes a hard exterior that won't cook as easily on your griddle.

Instead for home fries I prefer to blanch my potatoes in boiling water. This makes them soft and absorbent to seasoning and the starch is pulled to the surface which then gets that nice crispy exterior that you want at the END of the homefry cooking process.

Oh and since your size will effect your cooking time I will just give you a ballpark 7-10 minutes but really you are looking for about 60-75% doneness.