How do you calculate the extra depth the tadpole gives you?An invaluable tool to helping an angler figure out how deep a crank will run is the Precision Trolling Data - they have both stickers for your tackle box and an App for both I-phone and Android.
Here is an address for more information about the PTD: PRECISION TROLLING | thenextbite.tv
If you want to get a crank down deeper - I would suggest trying Off Shore Tadpoles - the #1 size will do just about everything you need as far as depth. The Tadpole is an inline diving weight that releases when you have a fish on so you fight the fish and not the weight
Here is an address for the Tadpole: Off Shore Tackle | Official Web Site OR36 1 Resettable Diving Weights
Here is the dive curve for the Tadpole: Off Shore Tackle | Official Web Site OR3