@Ray Jimenez,
The best advice I can give you is to have some general numbers on hand with you when you go to view a property. What the cost per square foot to repair this or that will be. Such as roofing, tiles, drywall, etc. Once you have those numbers in hand, and you assess what needs done and how much of it needs done, you can add up how much in renovation you will need to account for in the estimated repairs part of your deal.
After you know how much your repairs will cost your end buyer, and you know how much you want to make for your assignment fee, you need to take those numbers and subtract them from the ARV. If you know who you are assigning your contract to, or what type of return investors in your area like, you will know how much they need to make off of the deal. Once you have that info, you know how much you can contract the home for in order for all parties to be satisfied.
When the seller gets anxious at you low balling them, calmly explain how you arrived at your number. I like to start about 7k lower than what I actually plan to spend. If they reject your initial offer, step in up just a touch, possibly ask them the lowest that they could take for it. One technique you can use instead of offering more money, is offering a shorter contract period. Start at 60 days and knock it down to 45, then 30, if you are confident you have buyers waiting in the wings to jump on the property. A motivated seller, or someone selling an estate home, always loves faster money.
I hope this helps.