I agree very much with this. As far as the min range for titans it would probably be best to measure from the feet like Chase said is a possibility otherwise people might customize titans to lean over so they can shoot closer to their feet.
I'd be content with measuring titan's range from their feet.
Changing rules is far too much a slippery slope. Measuring from the feet require the shot to originate from the feet as well.
How about we not modify the 40k rules much more than necessary. The measuring from the feet changes the way TLOS works. Since 5th edition relies on it, wouldn't that be a little odd?
I guess if we are measuring distance and LOS from the toes, we can resolve that every weapon but D would give people cover saves. Also, guardsmen would give the marines behind them cover saves from the vulkan megabolter mounted on their carapace...
OR
We could use 5th edition rules.
Measure distances from the gun.
Use TLOS.
Problem solved.
Now for the explanation as to why it is solved:
TLOS basically forces the gun to be aimed downward. An emperor titan / warlord cannot be made to be leaning over and look "badass" or "cool" So the only real way it can aim is slightly downward or straight forward. We allow for the 45 degree angle swivel up and down and suddenly the rules resolve themselves.
We can do the math if you like but it's pretty obvious without trig to know you won't be able to shoot at the titans feet with carapace weapons by the rules as written.