Adding old sand

[ensquire]

I just completed a tank swap from my 36 gallon to a 25 gal to make way for an upgrade. I added new sand moved the rocks over and have moved everything alse now. Just need to know how much old sand should I use to seed the new sand?? I have a 2 inch deep bed in the new tank and will keep old tank running until I get the sand out .

Thanks