I have both of my CS's and my 96 GT with Hagerty and they are reasonable in determining the value.
I would go for more than the NADA. They have agreed value and don't hesitate to increase the values where it's valid. They even recommended I raise my value this year without my asking.
Keep in mind what the car is worth and if something happened to it what it would take to replace it. The agreed value of insurance on my 2 CS's is in the high 20,000's and low 30,000's with no questions asked. Both values were increased by them from what I was going for.
My 96 is a special policy and they insured me higher than what I thought it was worth as well. Their minimum on these policies are usually $15,000. Edmunds, NADA, Kelly's, etc have my 96 at around 7,000-8,000. They weren't sure it would go through since it was under $15,000. They called me back and said they use "Old Car Price Guide" and they said they would insure it for $12,000 if I was OK with that. I checked and Old Car Price Guide had my car at $12,000. That's a little over what I was trying to sell it for last year.
The extra value added very little to the cost of the insurance.
I found their prices very reasonable ... I'm paying about $650 annually for all 3 cars ... No restrictions (mileage, etc) except that I need to own another car as a daily driver ... and also includes roadside for ALL my cars, not just my Mustangs insured with them which was about $80 per year so insurance is around $570 without that.
You have Hagerty so you probably know most of this already. All you need to do is ask about the value you want. Worse case is they say no. I never thought they'd insure my cars for what they did but they went higher than I expected on all of them. I've been pleased with them overall.