The problem isn't so much the actual octane rating, but the consistency of the fuel.
Once you start mixing with pump gas, you're not going to know how much ethanol is in the gas and your jetting will vary from tank to tank, regardless of how precise you mix the pump gas and race fuel.
It's NOT recommended unless you get a large quantity and jet for that load of fuel only.
Once you run out, buy another large quantity and check your jetting or re-jet accordingly.
Only buying 5gal of pump gas at a time is a great way to inadvertently get too much ethanol in the mix and melt a piston.
Most of the time there's no way to talk a guy out of going down this road until something breaks, so all I'm going to say is KEEP AN EYE ON YOUR JETTING!