Don't assume that someone at your ISP can't tell you this information. You may be right, but in all honesty, they are the people who will have this information. Since you could purchase a static IP from them (even though you don't want one) they are the ones in the know.
You can't reliably narrow down the range... at all, really. I mean, let's just throw numbers out there. For example...
If your IP address one day was 126.96.36.199... who is to say that your ISP has only leased 69.79.10.xxx for use for their customers. They could have 69.79.xxx.xxx available to them, which means that there are more than 65,000 addresses available there. And if you monitor the addresses to which you are assigned for... say... a week, who is to say that these are the only subnets they operate in? Do you have to watch for a month? What if they rotate subnets every 6 months or every year? This is why I say you can't reliably guess at a narrow range.
Of course, this really depends on how you define "narrow" and "reliably" for the purpose of this.
This is why I say you should take the chance and call your ISP tech support. Tell them what you need and exactly why you need it, and ask to speak to a level 2 tech or a supervisor. Honestly, they are usually far more supportive than you would think, especially when they are telling you things that do not make them directly responsible. They won't be as helpful telling you how to set up your router (because they don't want to be liable for equipment they don't own), but giving you the range you can expect your IP addresses to come from is not something you would not already experience on your own over time. They might turn you down, but hey... you never know if you don't ask.