The automotive antifreeze gauge is calibrated for ethylene glycol. Most likely you'll be using propylene glycol in your boiler. It would get you close but it would not be accurate. Commercial antifreeze lens usually have an inhibitor package in them that could slightly affect the specific gravity of the solution.
Go on the Internet and look for charts that would give you the specific gravity, freezing point, boiling point, specific gravity's of solutions for both propylene ethylene glycol. Then you just have to convert from the automotive antifreeze gauge reading to propylene glycol.
Here's two links to start . . .propylene glycolethylene glycol