It's easy to reset your trip counter to zero every time you fill up.
In my case, a CX with the small tank, I start looking for gas every 100 miles, and better do it before I hit 125 miles.
I don't know why you would want to bother installing a gas gauge mod but if you do I will follow that build with interest.
I don’t recall anyone here ever doing that, so you may be entering uncharted territory. The CX Es and Turbos have fuel sensors/gauges, but those tanks would be costly and not fit without modifications. Maybe someone will know of a “kit”.
You'd think they would at least include decent hose clamps with a kit like that.
The subject of adding a fuel gauge has come up a number of times over the years but I don't recall anyone actually doing it.
Personally, I'm in the reset the trip meter when you fill the tank camp. My GoldWing has a fairly accurate fuel gauge (when I installed the Danmoto instrument panel on I tweaked the sender so that the gauge starts to flash just as I need to switch to reserve) but I still go by the trip odometer because I'm far more interested in how much how farther I can go before filling up than how full the tank is.
Whether I use a gauge or the trip odometer to tell me I need to know how far it goes between filling up and switching to reserve and it's a lot easier to subtract the current trip meter reading from that than it is to work it out from what the gauge says.
BTW: Welcome to the forum. Please add your location and your bike's model and model year to your profile so that you don't have to remember to tell us every time and we don't have to keep asking when you forget (see Forum Settings link in my signature).
And welcome to the world of antique vehicle ownership (they own us, not the other way around). Your bike is about 4 decades old and may or may not have had all of the maintenance necessary to keep it safe & reliable so it is highly recommended to download the Factory Shop Manual for your model (available through the CX Wiki - link in my signature) and go through all of the service procedures, regardless of whether your bike has reached the specified mileage.
I also recommend looking on all rubber parts with suspicion because rubber does not age gracefully. Check the date codes on your tires and replace them if they are over 5 years old no matter how good they look & feel (old rubber simply cannot flow around the irregularities in the asphalt well enough to grip, especially if it is cool or wet). If your bike still has the original rubber brake line(s) (should be replaced every 2 or 3 fluid changes = 5 or 6 years) I recommend shopping for modern stainless braided ones (they last practically forever and double the life of the fluid). And don't forget things like the rad hoses and the boot between the engine and swingarm (they can crack on the bottom where you don't see it).
If a person was skilled enough to weld on a tank and didn't mind having to repaint I bet there's a way to fit a fuel gauge from a Kawasaki fourwheeler. It's a very simple design I've been staring at for almost 20 years on my dads fourwheelers. ATVWorks Products Detail